First, welcome to Fish Lore
5.7 liters is something like a 1.5 U.S. gallon tank. I am sorry to bring you the bad news, but I honestly don't think you can keep any fish in such a small tank. You may have seen Bettas in your local fish store, sitting in even smaller containers - literally in small cups of water. But the truth is that these Bettas are only suffering there. It is very cruel to keep fish in such small containers. And although a 1.5 gallon tank would certainly be better than a small cup of water, I still say that even a 1.5 gallon tank is too small for a
betta as well. In my opinion, a 1.5 gallon / 5.7 liter tank is really too small for any fish. But, of course, you can do whatever you want with it. I guess the smallest fish being sold are fish like Neon Tetras, and similarly sized tetras. But the problem is that most Tetras (and most other small fish species) need to be kept in groups in order to be happy and healthy. A 5.7 liter tank is way too small to house a group of small tetras. I guess you could have, for example, 1-2 Neon Tetras in there ... BUT that would not be healthy for them. Like I said, most small fish need to be in groups, and I also believe a 1.5 gallon tank is way too small for fish-keeping. That is: for healthy fish-keeping.