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- #31
Interessstiiinngg . . .Gads, is that "rule" still making the rounds? I heard that when I was a kid!
The problem of how many fish a tank can safely hold is not a simple one. There are a number of limiting factors, including the amount of dissolved oxygen in the water and the ability of the biological elements in the system to safely process the waste. Another rule I have heard is based on the number of square inches of water surface (has to do with the ability to absorb oxygen out of the air), but the ability of the water to hold oxygen changes with the temperature, and moving the water with a filter and/or bubbling aeration can boost the the rate at which the water can absorb oxygen. Another rule says "one inch of fish per gallon;" the idea being that larger fish need more area, but that doesn't really cover the situation, either. If you think about it, one 6" Oscar has a great deal more mass than 6 inches' worth of Neon Tetras, and would generate a lot more waste and probably need a lot more oxygen.
And then you get into the actual oxygen requirements of the particular fish. Cory cats get part of their oxygen directly from the air (have you seen your Cory's dart to the surface for a fresh bubble?), so they can tolerate water with a lower oxygen content than some other types of fish. Labyrinth fish get all of their oxygen from air, which is why pet shops can keep Bettas in those tiny little bowls like wine glasses. Goldfish require quite a bit of oxygen, and cooler water can hold more, so goldfish that are perfectly happy in their tank at 60o F may be gulping air at the surface at 90o.
There just aren't any hard-and-fast rules about how many fish you can put in a tank. Basically, it boils down to maintenance. As I said, filtration and the use of airstones can boost the ability of a given amount of water to absorb oxygen. But even a tank that has water moving like a mountain stream still has only so much water in it, and the waste produced by the fish has to go somewhere. That's where the water changes that Rowsdower was talking about come in. When you remove water from the tank, you remove the waste products that are dissolved in it. Fresh water doesn't have the waste dissolved in it, so adding new water effectively dilutes the amount of waste in the water in the tank as a whole. How much waste the fish in the tank produce will determine how often the water needs to be removed and replaced to keep the amount of dissolved waste at a safe level for the fish. More fish = more waste = more frequent water changes. Of course, you want to make sure that the water you are adding is safe for the fish - water from some sources may need certain types of treatment to remove harmful elements before it is added to the tank, or it can be just as dangerous to the fish as the dissolved waste.
its a shame because i love them. i have probably spent upwards of 150$ on all diff. varieties of cory cats, pandas leopards etc. and could never get ANY of them to live. brought water samples to the pet store and everything checked out, they could not tell me why they were dying on me. I have a 30 gallon tank, primarily filled with guppies atm and a few tetras. really want some algae eaters, all of my miniature placos cant seem to make it but for about 3 months before they die.
