III. Old Tank Syndrome, OTS

What is it, and what do we do about it?

At the other end of the line from “New Tank Syndrome” (NTS) is its opposite, “Old Tank Syndrome” (OTS). OTS can take several forms, a couple of which we will briefly examine in this article. I would not expect OTS in any tank less than 12-24 months old, but I suppose it could happen if little or no routine upkeep were provided, or if the tank were grossly overstocked or overfed or inappropriately fed, or some combination of those things. That does happen.  OTS is most often seen after a tank has been operated 2-3 years or more – sometimes much, much longer.

The classic form of OTS is seen in long-established fish-only (FO) tanks with moderately stable populations. Both old goldfish and old Cichlid tanks are among the commonest, but OTS can occur in an old community tank or any other setup. These tanks have been operated as “business as usual” for long periods without disease or crash calling for special attention. This means the owner/operator is content with the setup, or perhaps just bored or inattentive and has not done major re-building of the tank or its accessories. It is likely that the owner has maintained an arbitrary – generally small percentage water change schedule, and may have become less than careful about keeping to that schedule. Historically the problem would most often have been detected due to attempted additions of new livestock to the tank. Perhaps a school of Tiger Barbs, or Cardinal Tetras (or whatever schooling fish was used) may have been reduced in numbers by slow attrition as the members of the school died, seemingly from old age. The owner wants to restore the school to its former number and buys some new fish. Or some long-time tank resident has died and the owner wants another fish to replace it. Note that the tank does not show any disturbance of nitrogen metabolism (no ammonia or nitrite positives with test kits), does not cloud up with either green or gray haze (although the water is often tinted yellowish), and that there is no obvious aggression between the new and old fish. The old fish remain fine, but the new fish die. This is the prime symptom of Old Tank Syndrome in fish-only tanks.

The condition here is the result of gradual changes occurring in the water over a long period of time. Because these changes were so slow – many weeks to many months or even years – the existing fish adapted to the changes without acute problems. The nitrate (and the other harder to detect pollutants such as dissolved organics and some mineral concentrations, the total dissolved solids or TDS) and perhaps the water hardness have gradually climbed to excessive levels, while the pH has likely drifted downward significantly, but has not acutely crashed. The slowly adapted existing fish are still alive, but to new introductions the water conditions or parameters are a shock to which they cannot quickly adapt, and they die. If the owner now tests nitrate (depressingly few people even own nitrate test kits), pH, GH, KH, they are (or should be) shocked at the difference in the tank and the tap water. The immediate and seemingly rational response is to do a massive water change to clean up the tank. After that, the existing fish start showing symptoms and or dying. If the new and short-lived fish brought in any infectious disease, it may now explode in the old fish. Even without disease introduction, after a massive water change the old fish may die from the same shock process as the new fish suffered in their initial introduction. The old fish cannot adapt to the too-fast change in the water conditions to which they had adapted over a long gentle period. It may seem a paradox for an improvement in water conditions to stress or kill fish, but it is not really the improvement, it is the rate of change away from conditions to which they were adapted.  The fish do not know or read this as improvement, but only as sudden large change.

All this may be seen as a long-winded argument for testing nitrate, pH, GH and KH at some reasonable interval, even if everything appears to be fine in the tank. And it is just exactly that. Those test results should be compared to the tap water, or to whatever modified water is used for the tank’s partial water changes. If this has not been done historically, and if there is more than a small difference, do not do large-scale water changes. If you do large-scale changes too quickly, you can push your fish into showing OTS, or even dying of it. Do repeated small-scale changes, starting at say 10-15% maximum. But do them daily or every other day (better, because slower) so long as your fish do not appear stressed. Monitor the decrease in nitrate and/or GH, and/or the increases in KH and pH. Then start moving the percentage changes upward as the tank water approaches the readings seen from the tap or other make-up water. Once you effectively match the tap, you can re-determine the proper water change schedule to keep the tank from buildup of undesirable dissolved materials in the future. But please do also remember that a tank when “young”, with juvenile fish, requires one level of changes to maintain stable conditions. A mature tank with the same fish fully-grown will need a different and likely a larger volume change to maintain the same stable conditions. If you do not test, you will not know. And what you don’t know can hurt you and your fish. A possible schedule for OTS clean up is suggested in a separate note on OTS Water Changes.

In a planted tank, the situation is more complex, as we do not have the same nitrate concentration to judge the pollution levels. Plants utilize the nitrogenous waste materials as nitrogen sources, so we expect our nitrate levels to be reduced in relation to the same stocking in a fish-only tank. If the tank is brightly lighted (equal to greater than 3 watts per gallon normal output fluorescent light) and heavily planted and fertilizer plus CO2-supplemented, we have to add nitrates to feed the plants. Other metabolic wastes may or may not be removed by the plants and bacteria in the tank. For me, I do the same water change schedule, but perhaps a bit smaller than in a similarly stocked FO tank. If I would do 33-50% weekly in a FO tank with those particular fish, in a planted tank I might do only 25-33% weekly. But I have to note that I do not often run high light and routine supplement tanks. When I do, those tanks get ~50% weekly partials to reset the supplement levels downward without massive testing requirements (essentially EI, Tom Barr’s Estimative Index handling).  In practice I tend to do 50% partials on all my tanks, regardless of whether or not they are planted.  To me it is easier and no doubt better for the tank.  Water changes done routinely and regularly, are never harmful to the tank.  Folks who say otherwise are admitting that they do not do enough water changes.  That means that the fish can detect the difference in the new and old water and that it enough to cause mild osmotic shock symptoms.

Planted tanks can have their own special version of OTS however. This to me seems to be related to buildup of organics in the substrate, especially in rapid-growth high light routinely supplemented tanks. Plant roots, like plant leaves, have a life expectancy. The faded leaves we remove at routine cleaning. What about the roots? Have you ever moved a 3-5 year old sword plant? There are massive dead roots, largely crowding out any substrate in the area under their foliage. There may be areas where hydrogen sulfide (H2S) has been generated with precipitation of iron sulfides (black) with the characteristic smell of rotten eggs from the sulfides. The plant is commonly doing fine despite this, and the tank will not have collapsed or crashed, the water parameters should still be essentially normal by hobby test kits. But there may be unexplained deaths of tank residents, not massive wipeouts, just occasional deaths of inhabitants, especially bottom dwellers such as many catfish, loaches, and dwarf Cichlids. Here again, these deaths are more common in new introductions rather than older residents. I can’t explain that last observation, as the toxic materials here should affect both equally. Perhaps the existing residents have located in the least affected areas, leaving only the more polluted spots for newcomers.  The fact that such events happen most often to fish that live close to the substrate and not infrequently disturb the substrate cannot be a coincidence.

The tank maintenance process for avoiding this is to keep the substrate “young” (just as we do for the water) with low organic loading. Cleaning and perhaps rebuilding the substrate periodically does this. When a stand of plants is overcrowded, don’t just pull out excess around the edges. Remove the whole stand, and then vacuum the substrate to get rid of dead and decayed roots. Then rebuild the substrate with whatever additives are desired, and replant at reasonable spacing. Alternately, for densely growing plants, take “plugs” of the stand (I use a gravel vacuum without the hose attached, like cutting cookies or biscuits). I space the plugs about one diameter apart, then vacuum and re-build the empty spaces and let them fill in again. My standard for all this varies from one to three years for most plants. Large swords I leave longer, but areas or segments of their roots may have been cleaned up without tackling the entire mass. Crinums are the one plant I do not disturb, ever if I can avoid it. For me they respond poorly to disturbance at the plant itself. But Crinum roots spread throughout the tank and are subject to renewal/removal when other plantings are renewed. Val needs renewal yearly at least, or it may start dying out or growing less vigorously for me. This renewal is for the benefit of the plants first and foremost, but it is also avoiding substrate issues that may affect the fish. I never redo more than ¼ of the tank area at once, usually less. But by the same token, no area is left untended indefinitely other than the few square inches immediately around a Crinum.

This is one of those cases where prevention is a minor chore, but is beneficial in multiple ways. The plants are healthier, the fish are healthier, and the tank is at much lower risk. Not too long ago, on another forum, a tank wipeout was reported from a gravel disturbance in a long-term “stable” tank. Hydrogen sulfide gas (rotten egg smell) was released and noticed by the hobbyist, and many fish were poisoned. If you do have a long-term tank (years) with a completely undisturbed substrate, do not try to do a major rebuild with the fish in the tank. The risks are too great.

New Tank Syndrome (outside of cycling problems) may be sometimes unavoidable in certain aspects – cloudy water can happen to anybody. Old Tank Syndrome is a self-inflicted wound, not intentional, but due to complacency or inattention or invalid assumptions. In biological systems it is not safe to assume that what you can’t see can’t hurt you. It can. But with a bit of extra monitoring or care on the owner’s part, it is avoidable. And if it turns out that both your tests, your plants and your fish always show that you are doing your job fully, you can lean back and savor the quality of your tank care. A bit of smugness and self-satisfaction is not an awful thing, so long as it is kept reasonably private and not waved about on the boards to the embarrassment of those first meeting those lessons that some of us have already learned the hard way.  Somehow, we don’t remember and mention our mistakes as readily as our successes.  But those mistakes can be our best teachers.


This note originally was published in AquaSource Magazine.  It has been edited and updated substantially for this site. Sept. 2010


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

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.