If you haven't yet experienced the problem of an overgrown tank, remember that prevention is the best medicine: read about whichever species interest you before you buy any fish at all. Never hesitate to ask questions of the person selling the fish to you.

Perhaps you already have your fish, and now you're stuck. Like a lot of people, you may have purchased, for instance, a two- to three-inch velvety, spotted catfish. While many aquarium catfish are Corydoras varieties that may reach a maximum of three inches at maturity, you bought a so-called Plecostomus Cat, which can reach a foot or more in length! Maybe you brought home three or four colorful baby Oscar Cichlids because they were beautiful and showed a lot of spunk, but now they're ten inches long and still growing.

How can you tell if your fish are too big for the tank? Observe them. Once fish begin feeling crowded, they may start behaving abnormally; larger fish may get aggressive.

Generally, you need to provide one gallon of water for each inch of fish. The trouble is, even when tropical fish owners adhere to this rule, they tend to think of the present size of the fish when they are purchasing it, instead of applying it to their fish's adult size. So if you buy four three-inch Oscars, they may seem well suited to a 20-gallon tank; in fact, the tank may almost seem empty. However, eighteen months later, when they're each ten inches long, it will be an entirely different picture.

According to the inch-per-gallon rule, you have twice as many fish in the tank as you should.

So here's the only really good answer to this question: to keep your oversized fish healthy and happy, the only thing you can do is provide them with a bigger home. If you can afford it, you'll find that a larger tank can be stunningly beautiful and is actually easier to maintain because the larger mass of water is less prone to sudden temperature changes. Most importantly, your fish will be a lot happier here - noticeably so. They'll perk up and should start behaving normally. If you can't afford to get a bigger tank, or if other circumstances prevent you from doing so, the only solution may be to sell or give away some or all of your fish. If you have friends with appropriate space in their tanks, that's great - you'll be able to judge in advance whether the fish might be going to a good home.

More often, though, you'll need to go back to the store where you bought the fish. Tell the owner about your problem and ask if the store will buy back the fish or give a store credit. Don't be surprised, however, if the store won't pay you for your fish, or that they'll only pay a small fraction of what you think they're worth. The store may be overstocked already, or their experience may be that there are few buyers for a two-foot long fish. Never release your fish into the wild, be it lakes, ponds or streams. Non-native species either do not do well in that environment or they deplete the other fish species.

Usually, as long as your fish are in good health, you'll be able to find a tropical fish store that will at least agree to take the fish for free. Try not to think of it in terms of how much money you put into the fish - think instead of the enjoyment you derived from watching them grow.