You can add anywhere between 10 to 30 neon tetras in a 20-gallon tank. I know that’s a vast range, but let me explain. 10 is a safe number to begin with. But if you’re an experienced fishkeeper confident enough with overstocking, 30 is perfectly doable.
The ‘one inch per gallon’ rule is up for debate, but it can be applied for fish smaller than 3 inches without any complications. If we were to assume the tetra’s average length is 1.75, you could fit 17 (17.14) tetras in a 30-gallon tank.
For small fish like neon tetras, we can follow the “one inch per gallon” rule. So, my calculations say that you can add 3 neon tetras in a 5-gallon tank. However, I’d recommend against it. Tetras are schooling fish that at least need to be in a group of 6 to be comfortable.
We can use the ‘one inch per gallon’ rule for small fish that don’t grow over 3 inches long. So, going by the formula, the ideal number of tetras in a 3-gallon tank is 2-4. You can add 2 neon tetras that grow around 1.5 inches long. As for small ones like ember tetras, you can add 4.
Neon tetras are schooling fish. If you keep several of them in a small tank, they will just hoover at one spot. So, we’d recommend not adding any more than 6 neon tetras in a 10-gallon tank. If you really know your way around ‘overstocking,’ you can get away with keeping 7-8.