This rule is not definite. Many argue that 1cm/liter is grossly more lenient than 1in/gallon. Others argue differently. What is important to look first at the footprint of the tank. It needs to be long for guppies. Then, taking careful consideration of activity level, one can adjust the rule. For very active species, it is sometimes suitable to cut the number you would keep for an inactive species in half. Thus, 54 liters (15 gallons) divided by maximum size of fish (roughly 40 centimeters, or 16 inches for ten fish) is 1.45cm/liter or .94in/gallon for an inactive species of fish. This means that a slower moving fish occupies nearly a 1:1 level, not taking into consideration filtration, actual amount of water or movement. This is just a model. However, for a fast moving species like guppies or the faster danio, we need to cut this number by at least half. So your forty centimeters of fish should be less than twenty centimeters, something closer to five fish. If you look at the gallon to inch "rule", you see less than eight inches which gives you about four fish. This is why the inch per gallon and centimeter per liter rule are so very disputeable. Each one gives you different numbers.fm1978 said:
