One of the trickiest parts of designing an aquarium is figuring out how many fish you can keep in it. Most formulas fall short in the face of aggressive, large or delicate fish. Fortunately, tetras stay small and play nice with others, and some species are very hardy.
Inch of Fish Per Gallon Rule
This rule is simple: 1 linear inch of fish requires 1 gallon of water. However, the rule starts to fall apart with fish longer than 3 inches. Fortunately, many common tetras fit well into this size range. For example, according to this rule, a 10-gallon tank could hold a 10-fish school of 1-inch neon tetras. This rule assumes the fish are full-grown and not particularly aggressive. This rule works well with peaceful schooling fish like small tetras.
Surface Area Rule
Goldfish image by Lucy Cherniak from Fotolia.com
The surface-area rule takes a little more math. It holds that for every 1 square foot of surface area, your tank can hold one inch of fish. A more detailed version holds that for skinny fish, you only need 1 foot of surface area but for wider-bodied fish you need 20 square inches of surface area. Again, this rule rapidly breaks down for most fish but works great for small tetras (most of which are thinner).
Using the Formulas
Both of these formulas have some caveats. Most of the time, the 1-inch-per-gallon rule will work just fine for tetras, since most popular tetra species are small and peaceful. However, the inch-of-fish-per-square-foot-of-surface-area can come into play with unusually shaped tanks since this rule is concerned with surface area for oxygen exchange. For very tall tanks, or unusual shapes with very little surface area, you may want to turn to this rule.
Other Tank Mates
fish fight image by cherie from Fotolia.com
While you can easily calculate how much room your tetras need, you will also need to figure out how much water your other fish need. Heavy-bodied fish need more water per gallon. For example, a medium-bodied fish need about two gallons of water per fish. Additionally, very aggressive fish, like cichlids, need more room to establish territories. However, most fish that get along with tetras are also small and relatively peaceful.