The small size of the tetra fish might mislead you into continuous thinking that keeping so many of them in a five-gallon tank as practicable. We would be looking at two main factors that make this a terrible decision to make.
Size: The principle that should be adopted when it comes to keeping small fishes like the tetra is the one inch per gallon rule, this means that one inch of the tetra fish is estimated to occupy one gallon in the tank but make sure you only apply this when the fishes are micro in size. The average length of most tetra fishes being about 1 to 1.5 inches, it is not a great idea to keep more than 5.
Shoaling fishes: Shoaling fishes are simply fishes that live in groups which are referred to as schools, for social and safety reasons. A tetra is an excellent example of fishes that live in ‘schools.’ Although they are a relatively peaceful species, they can quickly get agitated or scared when they are few.
A school of tetra fish is a set o 6 together. A five-gallon tank allows you to keep only 5 fishes which is nearly equal to the school number. So the great ideas is 5
WHAT TYPE OF TETRAS ARE BEST?
There are basically two main types of tetras that you will see most often in pet stores. The classic neon tetra and the cardinal tetra. Neon tetra are the smaller of the two and will grow to about 1.5 inches, so would be better for a small tank. They have red and blue streaks that don’t cover the entire length of the fish. Neon tetras are very susceptible to water temperature changes, so a good heater is recommended.
The cardinal tetra looks a lot like the neon tetra, but grow a bit longer and the red and blue streaks span the entire length of the fish. Cardinal tetras are also a bit more expensive because of their superior coloration. They prefer warmer water of about 75 – 80 degrees Fahrenheit.
Black tetra are the hardiest of the tetra species so it’s okay to have on a starter tank after it’s been cycled whereas with the neon and cardinal it would not be advised to have with a new tank. Black tetra tends to grow a bit bigger in size and prefer larger schools so they are not the type of tetra you would want for a small 5-gallon tank.
The green neon tetra is a very petite little fish with a beautiful green-blue streak that looks great in a planted tank. Since they only grow to about an inch, they are a better choice for a smaller tank.
Anyway, use a good small filter to keep the tank healthy and clean.