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The Importance of Stability in Aquariums

<< Cephalopod Articles | By Dr. James B. Wood

I'm always amazed at the number of newbies, my scientific colleges included, who think that you can just replace evaporated water in a fish tank. Perhaps the best way to more clearly illustrate the points I'd like to make would be using the following hypothetical example. Sue has kept a community tank of Angelfish and Tetras for just over a year. She replaces water that evaporates but has never done a water change. Her fish seem healthy and happy, although she hasn't had much luck when adding new fish as they die within a short time. Charles also has a community tank of Angels and Tetras. Unlike Sue, he does 15% water changes every two weeks. Charles' fish also seem to be doing quite well.

While talking with Sue, Charles discovers that she has never done a water change. He explains that water contains dissolved salts and when water evaporates the salts are left behind. Charles points to the white residue on the glass of Sue's tank and says "See, we have got to get your water tested, Sue! "Sue, deciding to take Charles' advice, brings a water sample to her favorite fish store. While waiting for the test results, Sue peruses the books in the store and does a little research on her own. She discovers two things: A) most of the books on Angelfish and/or Tetras state that they like soft slightly acidic water, and B) all the books strongly push water changes. Just then, the store employee comes over and informs her that her water is hard, very hard—actually the exact words used were "liquid rock!" The clerk then muttered something about.... What was it? Neutrons? Nutrition, nitrous, nitronium? Maybe nitrite, nitrate or nitroglycerin build up?

Sue thinks, "I want what is best for my fish. I know that my water is very hard, and that these species like soft water. And the nitroglycerin build up can't be a good thing either. I'd better do a large water change." So she does a 60% water change that night. In the morning she wakes to discover that all her fish are dead! Needless to say, she isn't in the best of moods when she meets Charles later that day.

Charles is stumped, he has done 75% water changes before—like when two of his Angels paired off and laid eggs in his community tank. He moved the family into a new tank filled with water from the original tank. The original tank was drained at least 75% of the way down. (For accuracy in this hypothetical example, I should point out that Charles' Angelfish ate their eggs immediately after the move. However, they did lay another batch three weeks later and these they raised just fine.)

The above situation brings up several questions. Why did Sue's fish die? After all, Charles has done many water changes, including several that were larger then Sue's. If Angels and Tetras don't like hard water, why were they fine before the water change? If Angels and Tetras do like soft, slightly acidic water why did it kill them?

This gets us to the two important points I'm trying to make. The first is that animals prefer a specific range of environmental conditionsmdash;i.e., Angelfish and Tetras prefer slightly soft acidic water. If Sue had continued just adding water and not doing water changes, it would only have been a matter of time before problems manifested themselves. This first point, that animals prefer a specific range of conditions, is widely understood among aquarists.

Why didn't Sue have any problems earlier? After all, her water was very, very hard. And why did her fish die when the water quality improved?

These questions can be answered by my second point; a point which is often ignored or not considered by aquarists. The rate of and degree of change in aquarium conditions is just as important as the actual conditions. Although the water quality in Sue's tank was very bad, it got that way slowly, over months and months. Her fish survived because they had plenty of time in which to slowly acclimate to the tanks conditions. Sure, Sue greatly increased the water quality with her water change. She decreased the amounts of dissolved salts and NO3 and probably significantly effected just about every water quality parameter in a positive way. Isn't good water quality something we all want?

However, Sue also caused a rapid and massive change in conditions, and that is what killed her fish. Sue had a similar problem when introducing new fish. Even though she acclimated them, the fish went from normal water to very hard water in too short a time causing their untimely demise.

Why didn't Charles' fish die when he did an even more massive water change? Well, since Charles frequently does water changes, the water in his fish tank is only slightly different from the new water. Thus when Charles does a water change he does not change conditions in epic proportions.

All of this information is old news to reef aquarists who regularly add some rather harsh chemicals to their tanks in order to achieve the narrower range of environmental conditions that their charges require. Reef owners use dosers, a device that adds these chemicals slowly over a period of time, to introduce these chemicals. Thus, their tanks have really good water quality and stability.

Still with me? Good. All of the above is necessary background for the points I wish to make about a behavioral pattern that seems to happen more frequently than aquarists who don't do water changes.

We all care about our fish, invertebrates, spouses, and plants. If you didn't care about the majority of the subjects listed in the last sentence, you probably would not be reading this article. Occasionally our pets become sick, and as aquarists we want to do something to help them. If that something doesn't appear to work or doesn't work fast enough, in many cases we will increase the dose or try something else. We do not want to sit by and watch out fish, invertebrates and plants suffer.

Another hypothetical example. (Are you starting to think that maybe these hypothetical examples are not entirely hypothetical?) A friend went home for the weekend and left her window open in her dorm room. Upon returning she noticed that the water in her unheated two gallon fish bowl was very cold. Her Betas and Tetras were alive, though not especially active. She immediately mixed in enough hot water to bring the temperature back to where it "should be"—around 78-80°F. You guessed it—this rapid change killed the fish. What would I have done? I'd have simply shut the window. The tank would slowly warm back up. I would also monitor the fish closely over the next few days. Then I would do a water change.

Time for another "hypothetical" example—this time one with a happy ending. A friend noticed that his pH was down to 7.7 in his octopus tank, so he added a tablespoon of buffer. His pH immediately shot up to 8.7, and his octopus was noticeably stressed. Whoops! He should have only added a teaspoon, not a tablespoon. What would you do in this situation? This aquarist, on the advice of his fiance did nothing other then keep a close watch on the octopus. By morning the octopus was fine. Would I have done the same thing? Well, actually, no. I would have done a 40% or so water change. This would not reverse the effects of the buffer, but it would diluted the effects. (Doing a water change is my response to most things.) What would be the worst thing to do? Dump in some pH decreaser to rapidly lower the pH. Why? One rapid change is stressful enough, and two is at least twice as bad. Who knows what would precipitate as you turn your tank into a living chemistry experiment! The rapid increase and subsequent rapid decrease in conditions would adversely effect just about any creature. How to do this in the future? Add the buffer slowly over time—likely the overdose wouldn't have stressed the octopus at all if slowly dripped in over several days.

I've noticed that many new marine aquarists are very quick to stock their tanks after waiting an agonizing month for it to cycle. Often there are problems because the tank may be somewhat cycled but it can't handle the load put on it and fish die. Aquarists, understandably upset about the loss of their pets, then want to tear the whole thing down—buy this or that piece of equipment, and start over again. If that extra bit of whatever you bought to improve water quality works, the second time you set up your tank it will remove ammonia, the food for your fledgling bacteria population. It will then take even longer for a good population of bacteria to develop. (Hint: don't use protein skimmers, carbon, resin or any similar device while cycling a marine tank.) I've seen the same person do this process several times.

By not tearing the tank down and starting over, you get past what I call the "terrible twos" —that second month when Poof! things are supposed to instantly go from bad to good (but the fledgling bacteria population is just getting a foot hold and the chance of wild water quality fluctuations is still high, especially if the tank is quickly stocked). The reality is, the longer you cycle your tank and the slower you add new critters to it the better.

Always remember that slow, gradual changes are always better than rapid dramatic ones. Think about jumping out of an airplane without a parachute—you are just fine as you pick up speed as because this happens over a period of time. However, the deceleration when you hit the ground is a bit hard to acclimate to. Generally, a two-month old marine tank is more stable than a one month old tank, and a three-month old tank is usually more stable then a two-month old tank. After 3 to 4 months, assuming the bioload is constant, the difference is negligible. Each time the tank is broken down and restarted, the most turbulent and unstable time for the tank is repeated. In almost every case, not tearing down the tank and just waiting things out is the better response. Given time and a reasonable bioload, things will stabilize.

Medications are another common way that aquarists quickly and radically change the water quality in their fish tanks. Some of the negative effects of medications do not directly act on the fish. For example, some antibiotics kill all of the good bacteria in the water as well as treating the fish's disease. The decaying bacteria and waste from the tank's inhabitants that the normal bacteria population would usually eat decompose. This causes an ammonia spike in now what is pretty much the equivalent to a fully stocked new, uncycled tank. Some medications are safe to use in the tank while others are not. Many experienced aquarists remove the ailing fish to another tank for treatment regardless of whether or not the medication is supposed to be safe in the tank.

I realize that knowing when and to what degree to react is often a matter of experience. However, often doing nothing or doing something small is much better then making epic changes that kill with kindness. For example, when my angelfish appear sick, all I usually do is slowly raise the temperature a few degrees, from 78 to 80°F and watch them closely. This usually gives them an additional advantage over what they are fighting and doesn't stress them out.

The point of this article is definitely not that you shouldn't use medications or attempt to bring substandard water quality up to par. Rather, the point is to think about what you are doing, why you are doing it, and consider both the rate and magnitude of the change. Rapid changes in environmental conditions are just as deadly as bad water conditions. By doing several small water changes, Sue could have slowly brought her water quality back to an acceptable range. Simply shutting the window of my friend's dorm room would have caused the room and the tank in it to slowly warm back up. Remember, the rate of change matters!

Articles Reference

Wood, J.B. 1999 The Importance of Stability in Aquariums. Freshwater and Marine Aquarium Magazine. vol 22, no 4. Reprinted in: Wood, J. B. (ed) The Cephalopod Page (

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The Cephalopod Page (TCP), © Copyright 1995-2018, was created and is maintained by Dr. James B. Wood, Associate Director of the Waikiki Aquarium which is part of the University of Hawaii. Please see the FAQs page for cephalopod questions, Marine Invertebrates of Bermuda for information on other invertebrates, and and the Census of Marine Life for general information on marine biology.