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Why Do My Fish Die?

The hardest part of starting an aquarium for many people is being patient at the beginning. It is only natural, once your new system is in place and full of water, to want to populate it with all of the different, interesting fish you come across. Unfortunately, the old saying “haste makes waste” was never more true than in aquariums.

The Nitrogen Cycle

Like all animals, your fish eat and produce waste. Up to 80 per cent of a fish’s body wastes are toxic ammonia compounds. Removing this from the water is the most important function of an aquarium filter. In an established system, these compounds are broken down into nitrite (also noxious), then into non-toxic nitrate. The nitrate is used as ertilizer by the algae and live plants, which can then be eaten by fish, or it is removed through periodic partial water changes. This process of breaking down and oxidizing the toxic body wastes is referred to as the nitrogen cycle.

Biological Filtration

The most efficient and reliable method of removing ammonia from the water is biological filtration. In this process, two types of aerobic (or oxygen requiring) bacteria break down the waste. Nitrosomonas bacteria convert the ammonia to nitrite. Nitrobacter bacteria oxidize nitrite into nitrate. The most common filter of this type is the undergravel filter. Regardless of the type, all biological filters require an initial break-in period, because when you first set up your aquarium, the bacteria aren’t present - they must be cultivated. In a sense, your aquarium is “too clean” when it is brand new. If you add a lot of fish to your new aquarium without having these bacteria populations, the ammonia levels rise in your water until they’re lethal, and your fish die - that’s “new tank syndrome”.

An Inch of Fish

So where does this bacteria come from? It is present in nature whereverwater, oxygen, and ammonia come together, including on the fish you add to your tank. To get the process started, you need to add fish to your aquarium - enough to start the cycle, but not so many that the waste builds to a toxic level. A good rule of thumb for determining this initial population is “about an inch of fish per gallon of water”. For example, in a 10 gallon aquarium, you might start with (4) one and one half inch long danios, (3) one inch long tetras, and (1) inch long corydora for a total of “10 inches” of fish. Your Tropical Shop staff can help you select some hardier fish for your break-in.

Some other factors enter into this. The larger the tank you are working with, the less absolute the limit, while the smaller the tank, the more critical it is. That is because it is harder to pollute 100 gallons of water than it is two gallons. The larger the aquarium, the greater your margin for error, and the less rapidly the environment deteriorates. Also, one ten inch long fish will produce a lot more pollution in your tank than ten one inch long fish - they are not equal. The rule is meant as a simple guideline.

After you add your first fish, the ammonia level will start to rise. There is a lag time before the bacteria population begins to grow in sufficient numbers to convert this to nitrite (see the graph).

Nitrogen Cycle Graph For a more in depth explanation of this process and the original that this graph was patterned after, check out The Krib.com.

During this time, if the water looks cloudy or the fish act stressed, cut back on your feeding and lower the ammonia level by changing half the water. As the nitrosomonas population grows, the ammonia level will fall and the nitrites will rise. Then, after the nitrobacter population has reached a point where your nitrite level has dropped, you can add more fish. This whole process can take from two weeks to more than a month. That’s why patience are important. You can speed it up by adding some gravel from an established tank to yours, thus introducing the bacteria in large numbers.

Then What?

After you are through the break-in period and have stocked your tank, your maintenance is to do regular partial water changes - start with 1/4th every two weeks. Also, NEVER take your gravel out and wash or sterilize it - if you do, you will end up going through this break-in period all over again from the beginning.

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