From Shirlie Sharpe,
Your Guide to Freshwater Aquariums.
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A. In answer to the reader question:
"We have Beta's in bowls but I have wanted to set up an actual aquarium. I have purchased a 10-gallon and have read up on which types of Freshwater fish live well together. However, I cannot seem to find out how many fish should live in a 10-gallon aquarium. I am also wondering since we are starting from scratch, is it best to introduce one fish at a time or a couple at a time, and if it is to introduce one at a time, how much time should go before introducing another fish? Any help would be great!"

The rule you'll most often hear about is an inch of fish per gallon of water. However, full bodied fish such as Goldfish, require far more space than that. You must also take into account the shape of the tank, as the air exchange happens at the surface of the water. A tall thin tank has less surface area for the volume of water than a short long tank. So before you use that or any rule, read more about calculation methods.

10 gallons is not a lot of real estate, so you won't be able to keep too many fish in that aquarium. I'd stick to small, slim bodied fish. A half dozen schooling fish, such as neon tetras would be lovely, with a small bottom dwelling fish or two. Or consider keeping a small school of Danios and a couple of barbs. There are many options, just take care to avoid fish that reach an adult size larger than inch or two.

Adding the Fish
Small aquariums are a real challenge to startup, so much so that I don't recommend them for beginners. Ammonia and nitrite levels can rise to dangerous levels very quickly because there is so little water to dilute them. Add only a couple of fish to start. Once the ammonia and nitrite levels fall to zero, you can add a couple more. Wait a week and test the water. If the ammonia and nitrite levels are still zero, you can add a couple more fish. Better to take it slow and easy, then to lose your fish.

Stop back and let us know what you selected for your tank. Better yet, send a photo - we'd love to see your new aquarium.