I like fancy guppies, swordtails, Angelfish (a little agressive) and even the ugly little catfish types. That tank would hold a pretty good number of fish if well planted.

I just sent your question to our resident aquarist.

In the meantime, according to this article it depends on how much maintenance you are willing to do and what types of fish you select. AquaHobby

You can get away with a ridiculous number of Zebra Danios; hardy little school fish that look a bit like Tetras but are tougher and actually do well in a tank full of predators (they are quick, so they rarely get caught). If you like Tetras, try Zebrafish :)

Unlike @Adirondackwannabe, I never keep live-bearers. They seem shorter lived, problematic, and prone to disease.

It sounds like you’ve got a nicely established tank. What kind of filtration do you use? Do you use a heater?

I like to utilize the different zones of a tank. Very active fish that like the upper parts of the tank like zebra danios, giant danios, or hatchet fish (can be harder to keep) will add a lot of life to the tank.

Mid-level fish like phantom tetras, black neons, cardinal tetras, bleeding heart tetras (obviously, I’m a big fan of tetras. They get along very well and are very hardy.) Some of the barbs will also do well here (tiger bard, gold barb, rosy barb).

The lowest level of the tank should have some janitorial service. Corydorus, algae eaters, and an interesting catfish will help keep the tank cleaned up.

The number of fish depends on their size and how well they do in a community, as well as your filtration system. If I were setting up a tetra tank of that size, I’d probably have a target of around 25 fish (added gradually so as not to overload the filtration system).

@syz I thought zebra danios were mid-level fish. Mine were. And they got along well with our plecos.

@syz

I like your idea of having an interesting collection of tetras. I do use one large heater in the middle of my tank. I have an external fluval canister filter, which works beautifully. I never had much luck with goldfish, no matter what I did, but I’ve had these tetras for just over two years. I relocated them into the big tank when I decided to stop keeping goldfish. They have appeared to be so happy that I don’t want to move them!

What do you think about keeping some variety of shrimp as well? I like the look of bumblebee shrimp. I have also though about a pair of honey dwarf gouramis. I kept a pair with these neons in a smaller tank a year or so ago, and they seemed to do fine together.

Like @jerv and @SofaKingWright I like Danios and Gouramis. Both are egg layers and should be kept in schools, but @SofaKingWright said he only had a pair and they did OK. I love Pearl Gouramis, and Zebra Danios. A Pleco is a good idea also. They are algae eaters, and good for most aquariums. (not all, but most)