VIIII. Veggie Filters

Vegetable Filters

It is generally accepted that healthy plants have a beneficial effect on the water conditions in fish tanks. They absorb minerals including some metabolic waste products from the water, converting these into plant mass. So these may be considered a form of biofiltration. The other mineral ions that they absorb and use could be considered a form of chemical filtration. During their lighted period they adsorb carbon dioxide (CO2), produce oxygen (O2) and submerse plants release excess oxygen into the water, so are also a form of aeration. If the water is not oxygen-saturated, there is no visible evidence of this. In oxygen-saturated conditions, very small bubbles of O2 will form at the leaf surface and be released- this is the “pearling” plant tank growers point to with pride as evidence of strong healthy metabolism in the plants. In the dark they do use oxygen from the water (and release carbon dioxide), but they do not use as much as they produced in the prior light period. So if we accept all that, it is reasonable to want to have plants in our tanks, not just for aesthetic enhancement, or even for refuge for the fish, but for the benefits they can provide to the water quality and thus to our fish.

Some tanks, however, just are not compatible with heavy planting, or even with planting at all. Have you ever tried to keep plants with an Oscar beyond half-grown? Or Uaru? Or Scat? Many fish, either by behavior (Oscars and many Cichlids), or by being herbivorous (i.e., plant eater, such as Uaru, Pacus, Scats, and Goldfish, along with many, many others) may be or are incompatible with plants. Larger Puffers may not eat plants as standard diet, but will likely damage them by biting holes in the leaves while gathering snails, or Oscar-like, just because they do not want them in the way. Such fish may also consider plant-cleaning crews such as snails, shrimp, sucker-mouth catfishes, and Siamese Algae Eaters as either potential enemies to be chased from the territory (impossible in the confines of a glass box), or simply as sushi.

Such tanks can still have the benefits of being planted, even without having plants in the tank at all. Have you ever heard of the concept of a refugium? In short, this is merely providing a refuge where the plants (or whatever) can live, separate from the display specimens and thus are protected by this separation, but with water exchange between the two areas. There can be several benefits here – not only having the positive effects of healthy plants on the water, but also simply by increasing the total water volume. Remember that “the solution to pollution is dilution”. A number of public aquaria use this effect for some displays, adding a second linked tank to the public display tank to conceal equipment and to increase the operating water volume.

There are commercially available algal scrubbers that may provide similar function, but I have no personal experience with these units. I have done DIY algal “filters” or “scrubbers” for marine invert tanks, where their function is very much like that provided by vegetable filters in freshwater tanks. I have also DIY’d macroalgae refugia to the same end. Walter H. Adey and Karen Loveland, in Dynamic Aquaria, Academic Press, 1991, advocate such techniques as scrubbers and refugia for maintaining water quality.

None of this is really a “new” concept. My personal experience here dates back to the 1960s, when my first SW (salt water) invert tank was filtered by a DIY algae scrubber. The specimen tank was a 10-gollon tank; the algae scrubber filled a 20-long!

Physical Setup and Circulation

The simplest refuge for freshwater plants is just adding another tank, with water exchange between the two. The arrangement I have used most often has taken advantage of those commercial tank stands holding one tank at normal seated viewing level, and a second tank (usually of the same footprint but lesser height) below it. It is this arrangement that I will discuss as an example, but many others are possible.

If you have a 55-gallon display housing a near-adult or adult omnivorous fish specimen, plants are out of the question. Filtration by a large canister or similar equipment can guarantee undetectable ammonia and nitrites. Despite regular routine maintenance by gravel vacuuming and partial water changes, the tank’s nitrate level may be higher than desirable for long-term maintenance. You could perhaps most easily double the water change schedule and offer much cleaner water in that way, but this may not fit your schedule of free time available. You might benefit by the use of a vegetable filter in the lower tank. A 33XL (48” x 13” x 12”) happens to fit well beneath a standard 55 (or 40-long) on several of my stands, so this is the tank selected for this example. In addition to the upper tank’s regular filtration, add an overflow siphon/surface skimmer such as is used with wet/dry (W/D) filters, or a Vortex constant-level siphon set. Constant-level siphons may be a relatively simple DIY (do-it-yourself) project. These units maintain a near-constant water level in the tank on which they are installed. Regular siphons will not work in this application. They would drain the upper tank and overflow the lower tank to create a flood. These W/D overflow units skim off the water surface layer and move the skimmed water outside the tank by siphons. The Vortex unit does not skim the surface but draws water from lower in the water column. Outside the tank the units incorporate a standpipe (adjustable in the Vortex), commonly prefiltered by a sponge (not in the Vortex, but it may be added if kept clean by frequent rinsing – another valuable pollution export technique), to drop water to a lower level. Some overflows are crenellated, that is, they have squared notches around the skimmer rim. Others have smooth flat rims. I see little functional difference between the two overflow types. They maintain the water level at least at the level of the overflow rim or of the standpipe. When additional water is pumped into the tank from a sump, the water level rises to overflow (hence the name) into the surface-skimming unit. The steady state operation with water being pumped into the tank will have an upper tank level slightly above the rim of the overflow or standpipe (i.e., will show a positive meniscus).

The output from the upper tank overflow is directed into the lower tank, arranged so that the output does not blast into the substrate if the veggie filter uses any substrate. My commonest technique is to plumb the output with PVC pipe, feeding the output into a horizontal section of PVC or CPVC pipe. That horizontal pipe is capped on the end and drilled with large holes along it’s length, or is made up if a series of tee-fittings joined as a manifold. This can be mounted either just above the substrate along the back or end of the tank, or just about anywhere below the water level of the lower tank. In either case, the holes in this DIY “spray-bar” are aimed horizontally forward (toward the front or opposite end of the tank) or angled upward to promote circulation. Such assemblies are not necessarily cemented in the portion of the assembly within the sump or refugium so long as it is under the water. This provides flexibility for alterations, by being merely pressure fitted.

Return from the lower tank to the upper uses any reliable powerhead or pump which will work without excessive noise against the “head” of the system. “Head” is simply the vertical height difference between the water levels in the two tanks. In the paired tank system in this particular example the head measures 32” with the stand and tanks I am using. For this example, an AquaClear 50 powerhead (old 402) is sufficient for low-flow applications; an AquaClear 70 powerhead (old 802) would provide more turnovers for a higher-flow setup. A number of other hobby pumps are suited to smaller or larger set-ups, such as the Eheim Hobby Pump series. Please remember that the rated volume of the powerhead is determined at zero head and zero resistance. In actual application we will fit the pump with a prefilter sponge (which offers some resistance to flow) and be pumping against a 32” head in this example. Rated output of this pump is listed as 270 US gallons per hour. Actual measured output with sponge and head is down to approximately 100 gallons per hour, or only a bit over one total volume turnover per hour. My standard usage is between one and two total volumes per hour. Low load setups have used fewer turnovers – remember this biofiltration and chemical filtration, not mechanical filtration other than incidentally, high flow rates are not needed. Flexible tubing (Eheim seems to hold its flexibility well and resists crimping for me) carries the output to the upper tank for most low-flow applications. Some of my setups are hard-plumbed with PVC or CPVC (any with greater than 3’ head are hard-plumbed). The top of the tubing is fitted with a return fitting that instead of the usual inverted U-shape offers another bend, to release the water return horizontally just below the tank surface. The return is best set with the release just barely below the operating water level. In the event of power loss, air would be sucked into the tube to stop back siphoning. Some of my applications are hard-plumbed with PVC/CPVC (usually 1/2” ID for combined total volumes below ~100 US gallons). Release to the display tank from these involves a couple of 45-degree elbows, usually not cemented but simply pressure fitted so that the final horizontal water release is partly open to the air both under power and power-off circumstances. If another type of return is used with release fully below the water level, it should have a hole drilled just below the operating water level. That hole is a siphon break. In the case of power or pump failure, water would start back siphoning from the upper tank to the lower. Having the hole drilled at or above the power-off water level will introduce air to stop the siphon during power-off conditions. If the siphon break is below the overflow unit’s rim level, this siphon break will determine the volume drained down to the lower tank in power-off conditions. During setup, this component should be tested repeatedly. This is a critical component in overflow or flood avoidance. In operation, the return should be kept clean by brushing it externally and internally. Allowing the siphon break hole to clog will lead to disaster and flood. I never rely on one-way valves for power-off or pump failure protection. They suffer from failure all too often themselves. That learning experience may be expensive.

Designing and tuning your own custom system will depend upon tank sizes and the pump chosen. In any case, in the event of a power outage or pump failure, the maximum water level in the lower tank must be set low enough to absorb the difference between the upper tank level with the pump on and the level with the pump off. With the overflow I have in place, this difference is just about 3/8” in the upper tank, about 1/2” in the lower. The difference is due in part to the positive meniscus in the upper tank. This level is marked on the glass of the lower tank by a tape line. I also mark the normal (lower) operating level with the pump running. When topping up the system during pump operation, this latter (lower) level is the guide. Filling the lower tank above this level could result in overflow during any power failure. This too is critical to the operation of the system. You must allow for drain-down under power-off conditions. This is also the benefit of having the second, lower tank as close as possible to the same footprint as the upper. It is certainly possible to use a smaller tank below the display and I have done so often. But the change in water level between power-on and power-off will be magnified by a factor equal to the difference in the surface area of the two tanks. In this example, if a 15 gallon had been selected, its change in level from powered to power-off would be just more than twice as large from the approximately 48×13” footprint of the 55 to the 24×12” footprint 15 gallon tank.

The Filter Itself, and Maintenance

The notes above gave some of the reasoning behind the use of a plant refuge or vegetable filter, and a typical setup that I have used in several systems. Basically the lower tank is a simple sump, which could be used in a variety of applications, but this article concerns plant filters. Now we can look at the sort of options for planting that have been used, and some variant layouts. Some maintenance hints are included as well, and a summary of the concept.

Multiple options are available for the vegetable filter itself. The selection made will depend on your experience level with plants and what you want from the system. The easiest and simplest option is to use floating plants in the lower tank. In this format, you need no substrate. Just purchase a few bunches of floating plants such as tropical hornwort or anacharis, remove any rubber bands or weights, and let them float freely in the tank. These floaters may also be anchored to the walls by suction cups attached to threads wrapped loosely near the base of the stems. In my experience, two 40-watt broad-spectrum tubes will serve nicely for a 48x 13” footprint tank. If your display houses a “Big Nasty” fish, it might be a good idea to have the heater or heaters for the whole system located in the veggie filter as well. Lighting should be on timers. The largest benefit to the system will have the veggie filter on reversed light cycle from the display tank. That is, if you have the display on from noon to midnight, set the timer for the veggie filter for 11:30 PM to 11:30 AM. From this the veggie filter will be doing its job and providing additional oxygen to the display tank during its dark cycle. Even if there is no planting in the display tank, the reversed light cycle seems to favor the least change in pH (from CO2 uptake by the plants) during the veggie filter’s light cycle. If there are some plants in the main tank, having both tanks lighted at the same time will magnify the daily pH swing a bit. Obviously in some settings, a lighted tank would be undesirable during the night. It would not be a good system in a regularly used bedroom. One of my systems operating in this fashion is in a room that serves occasionally as a guestroom. Black-painted cardboard is kept in a nearby closet to cover the front of the veggie filter when guests are in residence. The back of the tank is in the adjacent room, so does not contribute light to the sleeping area.

I have some experience with growing plants, and I frequently have an enriched substrate and rooted plants in the veggie filter. If you select low growing, propagation by runner type plants such as pygmy chain sword or dwarf sag, or small crypts such as Cryptocoryne wendtii, you will get increase in plant numbers and usually the excess can be traded in to your local fish store (LFS) for credit. Once the veggie filter (with plants) is established and stable (a few to several months), you could even add a pair of Ancistrus or Peckoltia catfish or algae-grazing shrimp such as Red Cherry shrimp and harvest babies periodically. Some floating plant cover may be needed for the crypts, or fewer hours of light. A few, not a solid cover, of the bunch/stem plants can serve for this, or any of the small floating plants such as duckweed. Duckweed can be a pest, but when controlled by frequent harvest (by netting much of it out for composting, or feeding to herbivorous fish such as goldfish, mbuna, or rainbows), it too serves nicely as a water purifier. The plant most commonly used for water purification, water hyacinth, I do not recommended for tank growing. It is IME (in my experience) moderately difficult to grow indoors under artificial light. Plants selected need to be ones that you can grow easily and perhaps rapidly, so they will absorb material from the water. Unhealthy plants will increase water problems, not reduce them. Supplements or fertilizers and/or trace elements may be needed in the water column, but this varies with the source water, the individual system, and the particular plants selected.

Emersed Growth Systems

In this setup, the plant’s roots are submerged, while the stems and leaves are in the air. This technique allows the plants access to room air and in particular carbon dioxide, which may be in short supply in aquarium water. Several families of aquatics grow in nature either submerged or emersed, dependent upon the seasonal water level in the stream, lake, or bog. Commonly they show different foliage in the two conditions. For our purposes however, it may be better (easier) to use houseplants than aquatics. Emersed growth plants seem to extract more minerals and metabolites from the water than the submerged plants (with the possible exception of anacharis, which is a real nutrient sponge for me).

I have used two different setups for emersed growth veggie filters. One employs the same arrangement as the submerged growth system discussed previously. The veggie filter is below the display tank. In the simplest format, a grid of plastic “eggcrate” (a light diffusion grid work that looks like a three-dimensional graph paper grid, available from hardware stores or lighting supply stores) is supported on short sections of 1 ½ -2+” ID PVC pipe (pipe size choice may vary with the needs of a particular setup) lying horizontally on the bare tank bottom. Metal-free cable ties may be used to secure pipe sections to the plastic eggcrate. One corner of the grid is notched out to provide space for the return pump and a float switch. The float switch should be one that will shut off the pump if the water level drops to or below the pump intake level. This will avoid the possibility of burnout of the pump. Water from the main tank is released into the area below the grid at the opposite end of the tank from the return pump. The water level is set at or just above the grid. Multiple cuttings of Pothos are pushed into the grid with the cut ends at the tank bottom in the water between the bottom glass and the grid, the foliage in the air. The same two each 40 watt tubes are used as in the earlier system. The Pothos will grow quite rapidly and need regular harvest. A number of different common houseplants will grow well in this setup, but should be selected for plants that will not grow too tall for the restricted height of this tank. Spathiphyllum has been recommended for this use, and it works well, but it would need a much taller tank. A Spathiphyllum veggie filter is unlikely to fit beneath a display tank, due to the height of the plants. Canoe Plant, or Moses-in-a-boat (Rheo discolor), also has worked in such a setup. It is much shorter than Spathiphyllum and thus less demanding of height. Cover glasses are not used with this setup, and due to the increased exchange with room air, there is more evaporation of water from the system. This setup requires daily or alternate day topping up for me, due to the relatively small water volume in the veggie filter.

The circulating range in my tank room uses Crinum americanum in ordinary aquarium gravel as an emersed veggie filter. This species is commonly known as the Florida Swamp Lily. It grows at moderate light intensities adequately or strongly with higher light intensity. This plant is much too tall to be grown below the display tank. I have used 20 gallon long tanks with plain aquarium gravel as the substrate. These bulbous plants develop massive root systems and need division about every other year. For moderate light I use two-tube each 32-watt workshop type fixtures, for high light I use two of these workshop fixtures. These plants are great nutrient extractors. The oldest leave yellow as they are shaded and outlive their usefulness. Just remove them as another nutrient export. Under high light these plants bloom all summer and are highly fragrant with elegant blooms. The fragrance is too much for me, so I remove the buds or use the lower light option to avoid most flowering. Obviously the high-light option is the most effective nutrient extraction process.

One alternate arrangement is a tank or tray above, beside, or behind the display tank, with the water level above that of the main tank. Gravel-filled pots (or pots with bottom half gravel, top half long-grain sphagnum moss, not peat) with plants are set in the tray with lighting suspended above them. Water is pumped via powerhead from the display tank into one end of the tray and overflows by a spillway (similar to that on hang-on power filters) or by standpipe back into the display tank. Other than DIY epoxy-coated or commercial fiberglass trays, I have also employed Aquarium Systems’ Gemini or System 2 filter housings for this, but I am not at all sure that these are still available. In this setup, the empty filter housing sits directly on the tank, and much smaller pumps are used than those suggested for standard filtration with these units. Plants such as Spathiphyllum, grown in recycled grocery store berry baskets seem to do quite well in these setups, and even provide long-lasting blooms (spathes) above the tank. The suspended lights above the plants are kept on the same cycle as the tank for this system.

Note that emersed plants do not significantly contribute oxygen to the water. Also note that with the veggie filter tray above the display tank, the display itself is the drain-down tank in the event of power failure. To avoid having the water level in the tank visible below the rim during normal operation, I make the plant tray substantially smaller than the display tank. This reduces the display tank change in water level.


The addition of a veggie filter in no way removes the requirement for regular water changes, vacuuming, etc. It supplements those tasks. It increases the nominal volume of the system by the addition of the second tank – as does any sump system, and unless used for breeding suckermouth cats it does not add to the system fish load. In the example used earlier in this article, the system was nominally 55 gallons. It now is a nominal 88 gallons. Dilution alone should improve the water conditions, as it has reduced the bioload. If before a 33% partial was done weekly, about 18.5 gallons was removed and replaced. Now the partial will need to be about 29 gallons for the same percentage change.

Floating stem plants will need to be harvested regularly. Remove the oldest portions to discard, keeping the newer parts for replanting. Rooted rosette plants will need to be thinned out periodically. Again select the older, larger plants for removal (this time as trade goods, not discards) and keep the younger, more vigorously growing plants in the filter. This sort of division tends to cloud the water temporarily, so I do this work with the exchange pump off. Usually I do only disturb and replant about 1/4 of the tank in one session – this minimizes the disturbance to the filter function. This also avoids over-loading the LFS with trade-in plants of one type. Emersed plants such as Spathiphyllum will need to have younger plantlets pulled off the mother clump and replanted or given away. For this plant I keep the larger plants for flower production. Pothos I tend to decimate by scissors, replacing some old growth with younger cuttings. The Crinums are not divided as often, about every other year. But they do have massive and heavy root systems. Generally the whole tank is done at once as there is much root disturbance, and the residual gravel well rinsed before re-setting with a few mature bulbs.

I do usually have snails in a veggie filter, Malaysian Trumpet Snails (MTS) in a planted-substrate tank, ramshorns in bare tanks of floating plants or in the water portion of emersed plantings. In submersed growth tanks, Amano, Red Cherry, or other algae-eating shrimp may be needed for assistance in algae control, or either Siamese Algae Eaters (which are not small as adults, and also less interested in algae then as well) or small suckermouth catfishes – the latter are my personal choice. Algae control in this system is more to keep the plants healthy and growing strongly more than for aesthetics. Algae itself could be used to remove nutrients from the water, but removing algae is to me much more work and far less pleasant than keeping the higher plants in check. Always rinse the prefilter sponge(s) frequently. I recommend weekly rinses at least. Removing trapped debris before heterotrophic bacteria have digested it is a good water management practice in any tank.


You can employ plants to improve your water quality, even if you keep fish that are incompatible with plants. It just requires a bit of ingenuity and juggling on your part. A bit of DIY is frequently required, but no advanced techniques are needed. There are many variations on the ideas presented here, and the refugium concept has application beyond the use of plants. Refugia also have been used to house daphnia to control green water or bacterial cloudiness. My first experience with that technique was based on Pet Library’s Advanced Aquarist Guide by Feroze N. Ghadially, London, 1969. I believe that book is long out of print, but it may still be available in some libraries or used book stores.

One of the variations on the theme is how the overflow or constant-level siphon in arranged. There are several ways to achieve the same end, some of which are less intrusive in appearance, but a bit heavier in the DIY aspect. Wet/Dry type skimmer/overflows have some disadvantages- they almost require the use of feeding rings if any floating food is used (BTW, homemade feeding rings are easily made by use of a length of airline tubing joined into a circle by a tubing connector. Customize to suit your fancy by the length of tubing used). Big advantages of such overflows are that they do not allow any surface film to form on the display tank and they are available commercially.

Exactly what level of water exchange between the tanks is “best” is only one of the unknowns you will be facing. These systems are not common. They do work, but as with all plant tanks, each system is individual. Generalizations are still difficult. In the example used for the first description, substituting a 40-long tank (for the 33XL) carpeted with crypts and a few floaters could probably use a lower exchange rate. It would also need less care and attention. The same 40-long with Val or young Amazon swords might or might not be able to use a third light, but the same exchange rate as used for the free-floating stem plants. When you change any given component, it is going to affect the rest of the system. The same 40-long with rotala, hygro, or similar hungry, fast-growing stem plants would need at least the third tube, or CO2 supplement, or both to maximize nutrient uptake, and could possibly need macronutrient supplements as well as micronutrient dosing. So the selection you make will depend in part on the job you need to do. Overkill is a real possibility. Setting a veggie filter that requires weekly or biweekly pruning, high light, supplements and CO2 could be swatting a gnat with a bulldozer if the original problem was of relatively small scale. Try to set a system that will simplify your life, not complicate it. Playing with gadgets and tanks is entirely too much fun, IME. I have been guilty more than once of building a Rolls Royce when what I really needed was a wheelbarrow.

Very low nitrate tanks are possible with heavy planting, as is clean, highly oxygenated water, even when the display specimen is incompatible with plants. All you have to do is set a planted refugium, preferably without flooding your home. Hopefully you will be able to do this in a way that will lessen your workload and increase your pleasure and satisfaction with your fish, at the same time improving the fish’s living conditions and hopefully prolonging its life. This is definitely a win-win situation.

This article originally appeared on another site. It has been edited for this site. October 2010

Robert T. Ricketts, a.k.a. RTR


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Robert T. Ricketts

Retired research scientist (biochemistry and physiology, pharmaceutical development) and senior process analyst. Started fishkeeping in the dark ages (1950s), first SW tanks in the mid-60s, first puffers in the early 60s. Started with two tanks and never less than multi-tanked excepting some periods in college and grad school. Specialty if any would be filtration and water management. Primarily species tanks, planted whenever possible/practical and some where it not really practical. Ran something on the order of >150 tank-years* in studying optimum tank conditions for F-8 puffers, the largest tank study I have done. Other studies have been significantly less. Alternate canister use was mid-40s, OERFUG just over 60, veggie filters only about 25 to publication, but still going on less intently. If it had been known that the F-8s would live so long, it probably would not have been started at all. *One tank-year is one tank for one year.