How many tropical fish would you recommend to keep in a 34L tank?

glumshoe:

I’m no expert, but that depends on the fish. For instance, you could keep one betta or none pirañas.

Aqadvisor.com is a site that can be used as a basic stocking estimate. It’s not a guarantee, and you do have to research the fish in question to make sure they aren’t an active species that needs loads of space, but you can use it to get a general idea of what your stocking level is.

That’s about an 8 gallon tank. The only suitable fish are very small ones, an inch or less. If you pick fish that size, you could keep about 10 of them, if you keep up with the water changes and have some simple live plants like java moss.

My fish suggestions are below. All are schooling fish that get to an inch long or so. Pick two groups of 5, or one group of 10. 

Make sure your tank is cycled before you introduce any fish. Look up fishless cycling, fish-in cycling is inhumane. You’ll also want to get a water testing kit so you can check ammonia, nitrites, and nitrates. You should never see ammonia or nitrites in your tank except when it’s cycling without fish. 

Don’t replace your filter media. Rinse it gently in a bucket of tank water and then put it back in the filter. Your good bacteria live in there and in the substrate. If the power goes out, remove the filter media from the filter, gently rinse it in a separate container of tank water, and place it into the tank itself to make sure that bit of water in the filter doesn’t go stagnant and kill the bacteria. 

Suitable fish for a roughly 10 gallon tank:

Neon tetras

Glowlight tetras

Penguin tetras

Porkchop rasboras

Harlequin rasboras

Pygmy corydoras (prone to losses when freshly imported)

Dwarf corydoras (same) 

Guppies (all male. 6 female guppies would also work but you’d need to regularly remove the fry)

Chili rasboras and phoenix rasboras are extremely tiny, you could keep 20 of them in a planted tank that size, but they’d have to be kept alone so as not to be bullied by other fish. 10 of them and 5 of the cories above would also work. 

You could also substitute a single dwarf gourami for 2 of the above fish, or a sparkling gourami for one, but keep in mind that all schooling species need at least 5 members of the school. 

A betta may or may not do well in a tank with all of the above except the guppies. Some bettas are too aggressive, some are too easily stressed. A female betta would have a better shot. If you add a betta, make sure you have a suitable tank (at least 5 gallons, so about half the size of this one) to put the betta in if it doesn’t get along well. 

You could keep 5 or so African clawed frogs instead of fish. 

8 White Clouds are suitable, but are coldwater fish. Their water needs to be about 76 degrees, not 78 like the others.

Nerite snails are suitable for any of the above tankmates, and one or two don’t count towards your stocking level. You need to have the tank cycled for a month or so first to be sure there’s algae for them to eat.