Two kinds of question live here and they need the same opening details. One is "can I keep these together" — the other is "they have bred and I was not expecting it". Start with: Tank dimensions , not just volume. A 75-litre long and a 75-litre tall suit completely different fish. Everything currently in it , with rough numbers. "Some tetras" hides whether they are a proper group. Your water — hardness and pH if you know them. Some of the most common pairings only fail because of water, not temperament. For breeding: the species, whether you want fry to survive, and what else is in the tank. In a community tank the honest answer is usually that they will be eaten, and that is fine. Stocking answers are opinions with reasons attached. If two people here disagree, ask them both why — the reasoning is more useful to you than the verdict. One house rule, and it is the only one that matters: answer the question that was asked, and say when you are not sure. A confident wrong answer costs somebody a fish.