How many fish can you keep in your aquarium?

”…In any case, every attempt to quantify the number of fish you can keep in an aquarium is simply an attempt at formalizing an opinion.

You could visit the breeder’s home and witness how, in a small tank and with just a simple airlift system, fish can eat perfectly, not become sick, and even multiply. Sometimes, however, the opposite happens — the size of the tank seems to be good, water changes are done, and the fish become ill and die, and algae grow all around (This usually happens in beginner aquariums). 

General recommendations for keeping fish include matching their size, temperament, and temperature regime, as well as the type of food they require. However, what exactly constitutes “type of food” can be a bit of a grey area, as there are many factors that go into determining what type of food is best for a specific species. For example, some fish, like Botia’s and cichlids, get along very well together, as do catfish like Hypancistrus’es. Despite having different types of food needs, these three species can all be kept in the same tank. The key is to understand the nuances of each species' requirements.

And one more thing. Remember that fish are living organisms like everyone else, and you should never reason with the idea of “Well, if you don’t like it, I’ll pour you down the toilet”. If for even a second, such a thought comes to mind, it’s better not to even start an aquarium. Therefore, I repeat: begin by calculating how much nitrate and phosphate you accumulate by the end of the week, before changing the water. If you exceed these figures, then there’s an overpopulation problem. Plus, as I mentioned above, you also need experience keeping fish. Even if you read a lot, you still need to try out a lot of aquariums on your own. In other words, it’s not just in theory, but in practice. With a quick and easy solution, you can only maintain the most common types of fish. Apparently, this is why they’re so common. It is absolutely possible to give some examples of what not to do. 

For example, combining goldfish and scalars is a striking example. This combination is something I often see among amateur aquarists. If you are just beginning to keep an aquarium, a question for you: why can’t you keep scalars and goldfish together? To choose the right fish population for your aquarium, you need to follow these three rules: to choose the right fish population, you need to follow three rules: household logic — there shouldn’t be so many fish that the whole space is occupied by their fluttering, aesthetics and some set of tests for water…”

From my book “Guidebook of planted tank”