VII. Brown Algae, Diatoms In Freshwater Tanks by Robert T. Ricketts, a.k.a. RTR

Diatoms are very common algae (yes, they really are algae) in the world. They occur in freshwater, brackish water, seawater, soils, and damp exposed (emerse) situations. They have yellow-brown photosynthetic pigments, so most forms appear brownish to us. In tanks, mostly we see diatoms on lighted surfaces such as the tank walls, décor (rocks, ceramics, plastics, substrate, etc.) and as coatings on plants (either live or artificial). In freshwater (FW) they are generally unicellular, single cells, but in marine environments can have much more complex colonial forms.

Diatoms have a unique cell wall, made by them from dissolved (i.e., soluble) silicates into silica frustules or tests. These are two-valve shells, one half slightly larger and overlapping the edge of the other. Normally they multiply by division, with each daughter cell getting one valve of the mother cell’s shell and then producing a new half shell just inside the older half (See Note A below for more detail). They need the usual dissolved nutrients and light, plus dissolved silicates, all from their environment. Being photosynthetic, they are carbon-fixers (see Note B, below), so are contributors to the world’s supply of organic carbon as well as being minority contributors to the oxygen in our air (BGA, blue-green alga – really photosynthetic bacteria – produces more of our oxygen).

Both Seachem (1) and Salifert (2) make silicate test kits for hobbyists. I have no experience with the latter test kit, but have used the former without problems. It appears to be accurate and reproducible, but may seem unfamiliar to hobbyists who have not used multi-well dishes rather than test tubes or vials for testing.

Freshwater supplies, especially ground-source water supplies (deep wells from aquifers), tend to contain soluble silicates as the majority (more than three-quarters) of the Earth’s crust is made up of silicates and other silicon-containing minerals of highly varying solubility – many but not all are very poorly soluble. Silicates are compounds of silicon (Si) and oxygen (O). So in chem-speak SiO2 is the simplest format, but it is not an anion, just simple silicon dioxide. The true silicates are anions (with varying numbers of silicon and oxygen atoms and a net negative charge on the ion). They form salts with the metallic elements, which are the commonest mineral forms. Other than those native to the water supply, silicates may be added to the water in processing by utilities. Deep-source water supplies are likely to have fairly stable concentrations year-round. Surface source water supplies are likely to show more seasonal variation.

Silicates are not a required report by the U.S. EPA (Environmental Protection Agency), so may not be reported on the required utility reports to consumers. The local utility may well have the numbers and likely, in my experience, will share them with you if asked. The reason this is significant will be explained below.

All that background was the long, slow curve. Now the fast break:

What do diatoms and soluble silicates mean to you and your tank?

If your water has “low” soluble silicates (<1-2ppm for FW, in my personal usage), a freshly set new tank with all new water may well develop an unsightly brown film on the glass, rocks, substrate, plants (real or artificial). If you do not know what it is, you may well be horrified and a bit anxious. But diatom algae clean off easily – they are masses of tiny unicellular alga cells (which may even feel just a tiny bit gritty to sensitive fingertips when you wipe them off) – without any firm attachment to the surface on which they are found. So do just that, wipe them off. If you maintain good water quality, in time (weeks to a few months) the diatoms are likely to die back or even disappear. Diatoms require soluble silicates. Their “shells” or tests are not very soluble (they actually make up geologic strata formed in ancient seas, see Note B for more information), so are very poor recycling material in tank micro-ecology. The alga requires fresh-source dissolved silicates. With low ppm in the tap water supply, even large-scale (50%) weekly) partials are unlikely to be enough. There may well be relic populations in the dimmer areas of the tank (FW diatoms do not compete well with most green algae either, the green stuff out-competes the brown for the better light), but will not be a real and visible problem from diatoms anymore. If however you allow the nitrate, phosphate, dissolved organics, etc. to build up, both diatom and green algae will be more prominent or will be maximized, or more of a problem, regardless of the low silicate levels.

If your water supply is what I call moderate dissolved silicates, 3-4ppm, you will likely see persistent diatoms in the lower-light areas of the tank, or on the substrate where it is partly shaded by plants or other decor. To me this is not a major issue, and not often really that visible to non-hobbyists. The same warnings still apply about good water management and any algal form.

Some utilities may define levels up to ~10ppm as moderate. That approximate level of dissolved silicates is the standard for seawater. Scales of this sort are arbitrary, and I use a personal scale that correlates in my water with visible diatom algae in my tanks. For your water and light conditions, my informal usage may or may not be a good guide. If your findings do vary, please let me hear about them. There is not much decent data on silicates and the resulting diatom populations in freshwater tanks from within the hobby. Personally I would love to see a lot more. We could all learn from it. As a side note, reef tanks need even lower silicate levels than low ppm FW to avoid unsightly (and potentially harmful by shading) diatom coatings on the many photosynthetic organisms present. If you have seasonal variation in silicate titers (levels), you can expect diatom visibility to fluctuate with those titers. This is directly comparable to seasonal green water appearances resulting from fluctuations in both natural light and the dissolved mineral profile of the water supply, especially where the water source is surface water (such as rivers, lakes, etc.).

If your water supply is high in dissolved silicates, to me from 5ppm to 20 or even 30ppm, you will likely see diatoms anywhere in the tank not occupied by plants or green algae, and the plant or algae surfaces themselves may look brownish to frankly brown from these algae existing as a coating on the plants and even on some other algal types.

What can be done for silicates?

The easiest and simplest handling is just to remove the algae manually. If your water is low in dissolved silicates, that is very likely all that is needed. As the tank matures biologically, after approximately three to six months from stocking completion, diatom problems tend to fade. Should such tanks have some major biological upset requiring daily or alternate-day large-scale water partials, the diatoms may reappear. They should fade again when the tank returns to more conventional handling and upkeep.

For tanks with higher levels of dissolved silicates, manual removal is still the simplest path to follow, but does represent much more time spent on routine, boring to frustrating manual labor. There are alternatives. None is really simple or easily and cheaply applied.


There are creatures that relish diatoms. There are forum discussions regularly on the wisdom and philosophy of using fish or inverts for routine cleaning chores, but those are out of scope for this article, the choice is a personal one, so for this article it will be noted here and left as personal choice.

Otocinclus species catfish particularly relish diatoms, perhaps preferring these algae to other foods. In my experience, tanks with chronic diatom algae visible – not in great masses but always present – seem to make the best homes for these fish, especially if they are planted tanks. The Otocinclus adapt well in such tanks (if other tankmates are suitable) and live long uneventful lives. It the best of circumstances, they may even breed in such tanks.

There are several other members of the same suckermouth catfish family (Loricariidae) that also eat diatoms, but not with the specialization toward them that the Otocinclus show for me. A number of other fish also graze on coating algae including diatoms, even including the Lake Malawi mbuna Cichlids. But such fish are not at all likely to be selected as workers in a mixed tank – they are not suited to that role.

Several types of snails consume diatoms as well. Both common ramshorns and common pond snails do so, but neither is likely to maintain tank walls and décor well enough to be considered “solutions” to diatom problems. The best “worker” snails for this algae that I have kept are the Olive Nerites. They can actually control moderate diatom issues on tank walls and some décor and some plants. These are not huge snails, but more massive than common ramshorns, and with much rounder and several times heavier shells – their mass means they cannot clean small-leaf or fine-foliage plants well, if at all. They are equally effective in clearing soft green algae for similar locations. They do not produce viable young in freshwater (that requires brackish to marine conditions). The do however deposit individual eggs visible on the tank walls. They would be worth a trial in moderate levels of diatom issues, but these are not Apple snail sized critters, some numbers of them will be needed. I have too little experience with the Apple snail family to say how useful the various species might be against diatoms. There again, I would welcome input from other hobbyists

Several grazing shrimps also eat diatoms, but I have not had any that were specialist enough to control unsightly diatoms. To me their eating is rather like that of ramshorn and pond snails – yes, you can tell they are consuming some, but it is not enough to be useful.

Do remember that if you elect to use “workers” that it is your responsibility to be sure that they are suited to the setup and conditions, and most especially to the other inhabitants.

Reverse osmosis:

Conventional reverse osmosis, RO, systems will remove much of the dissolved silicate content, as it does with most other dissolved mineral ions, especially when operated at relatively high rejection rates of 4 or 5 or more to 1 – that is, 4 or more volumes to waste for each volume passed through the membrane to save. It would be best to test the product (the saved pass-through water) to confirm the reduction of silicate levels from those seen in the source water. Testing various rejection rates to find the most practical waste to product ratio is worthwhile. If very high rejection rates are required to clean the water of heavy silicate loads, RO may not be as practical. For this application, the product water almost certainly will need to have Kent’s R/O Right (3) and/or Seachem’s Equilibrium (4, with or without their Alkaline Buffer, 5, or other KH supplement as needed) to get GH, KH, and other mineral content back to balanced and stable levels. Liquid based test kits for GH and KH must be used routinely in preparing this water and checking the tank for stability. All water modification needs to be done and tested before the water is added to the tank. This does mean that extra vessels, pumps, heaters, and all the needed supplements and mineral replacements must be stocked.

Specialized absorbents:

There are absorbents available, such as Seachem’s PhosGuard (6) and Kent’s Phosphate Sponge (7), which will remove phosphates and silicates from water. There are others, but I have no personal experience with them. If you wish to try an alternate brand, be sure before purchase that the selected brand removes silicates as well as phosphates. As with other water modifications, these processes are carried out using vessels other than the tank itself to avoid osmotic and other water parameter shift stresses on the fish and bacteria in the operating aquarium. Both of these products may be used with tap or RO water. Tap water high in phosphates will require more of the absorbent to have significant effect on the silicates levels, but both can be reduced to or removed below hobby test kit detection levels. If the tank is a planted tank, phosphates must be replaced at the proper levels to balance the necessary NPK (nitrogen:phophorous:potassium) ratios.

These absorbents are generally placed in a fine mesh bag in a filter housing (either internally or externally prefiltered to avoid clogging the material with silt) to maximize water flow through the material. Fresh absorbent will remove the silicates within 12-18 hours in my experience, commonly faster. Full removal is determined by silicate tests. Spent material cannot be recharged, so must be replaced. The absorbent does not adversely affect the water other than by the removal of phosphate and silicate.

Obviously, as with RO, this process requires vessels and pumps and tests and time to prepare, so there capital and operating cost to be considered in either option. For planted tanks, phosphate supplement is needed as well.


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Author’s note: Unfortunately, I have no connection with the companies whose products are listed here.

Expanded information:

Note A: This form of division has strange results. One of the two daughter cells gets the large side of the shell, and produces a replacement smaller side of the shell, so is at least potentially the same size as the mother cell. But the other daughter cell, which got the smaller half-shell, produces a still smaller replacement half to fit just inside what it got from its parent. That process continues. This means that the average population member is getting smaller with each generation. That seems a sure route to extinction. But these algae have an alternative. When the cells hit a certain minimum viable size, they become sexual, and produce a much larger different type of cell that gives rise to standard sized cells, restarting the whole diminishing processes again. So this microscopic creature combines bacterial-style asexual division with a sexual generation to restore full size when needed.

Note B: Carbon-fixers are organisms that can “fix” atmospheric carbon from CO2 (or dissolved CO2 gas in water) into organic molecules. These are the aerobic critters that help start the food chain for the rest of the life forms here on this planet. This large group includes algae of all sorts, photosynthetic bacteria, etc. and all the higher photosynthetic life forms that we know as plants. A similar major base group is the nitrogen fixers, which may or may not overlap the carbon fixation group.

Note C: These geologic deposits, originally formed in shallow seas eons ago, are our source for diatom powder, which we use in specialized filtration techniques for very fine particle capture. There are many uses for this material in science and industry. These shells are not solid as are clamshells, oyster shells, or scallop shells which are more familiar to us, but have many tiny pores which in the living creature intake nutrients and expel wastes. In the ancient shells, many broken up by pressure and time, these microporous and the broken shell fragments provide us with micron to sub-micron particle capture.

Published by

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.