For most people, especially beginners, one inch of fish per gallon of tank space is a quite good rule. This means that the total of all the fish in the tank's full grown length equals the number of gallons. This isn't by any means a "rule" though. Read my theory on stocking levels. (http://www.geocities.com/shtinkythefish/stocking.htm) Note- some fish should never be kept in a tank equal to their length in inches. An Angelfish only gets to be 5 inches long, but should never be in a 5 gallon tank. Make sure you fulfill each fish's minimum requirements.
