A 20-gallon fish tank makes a nice display, but watch out for overcrowding. Adjust the number of fish to the size of the tank to keep everybody happy, including you.
A general guideline to follow when stocking your aquarium is that you can safely have an inch of fish per gallon of water. The advantage of this rule is that it’s simple to follow. The downside is that it doesn’t work for every situation. Adding 10 two-inch guppies to a tank is not the same as dropping in two 10-inch fish, but if you’re planning on setting up a community aquarium with an assortment of smaller fish, an inch per gallon is a safe bet. Just apply a bit of common sense as well.
When figuring out how many fish can live comfortably in your 20-gallon aquarium, you have to plan for their adult size, and not how big they are when you first get them. Many times the fish you buy at the pet store are youngsters, and they may get quite a bit bigger before they’re done. Find out what the average size of each fish is when it’s mature, and use that measurement to figure how much space it will need. It’s not unusual for a fish to double or triple in size after you bring it home.
Take the fishes’ behavior and temperament into account when planning how many to add to your tank. Fast-swimming fish need more space than slow swimmers, especially if they tend to be very active a lot of the time. You’re also better off giving aggressive fish more space, which helps to keep the peace by reducing the number of unpleasant encounters that occur. Also consider that a few bigger fish will create more waste than a bunch of smaller ones, and this can affect your tank’s holding ability as well.
Anything you add to your fish tank, such as plants and gravel, reduces the amount of water available for the fish. Then add a cute little castle or a lighted volcano, and you’ve made a real dent in the amount of water in the tank, as well as how much room the fish actually have for swimming. Boost the number of fish your tank can safely support by adding aeration and filtration, but don’t become too reliant on this assistance, since a power outage or equipment failure could spell disaster.