Como dizer "copular e cruzamento" em inglês

The fish is in the breeding/spawning season. (breeding would be used to other animals too, dogs, horses, etc - spawning eggs is for fishes and it happens in their breeding season.) And here, we are talking about fish in general (so, it´s 'fish', even when we mean plural in the sense of an animal species - aka animal category).
Breeding can be also be translated as "criar" (animais, por exemplo), so it goes with context. The sentence "

There were three fish in the tank (plural – all the same)
There were three fishes in the tank. (plural- three different species of fish)

Anyway, I saw a Q&A section of a scientific site where there was this beginning of a question:
I'm currently trying to breed zebrafish with for FET Test. However, the fishes are in the breeding tanks for almost 3 days now and they haven't spawned any eggs yet

Nobody complained so it seems there is not fishy about it... Dunno if it was because it had been mentioned zebrafish, but okay, we can live with that. Just let´s not do it in academical texts.

Your question made me think you meant "cruzamento" and "copular" in the same sense.
Sometimes there´s a subtle biological difference, it´s when "cruzamento" means "mix two breeds", something they do to improve breeds, to improve genetic traits (beautiful flowers/to have exotic colours in a given type of flower, etc).
With dogs, some "crossovers" makes the new dogs "mutts", sometimes a new 'sought-after' race, kind of.

Crossover with fish us not heard of, as far as I know, even because they are particular with their 'mating' habits, a fish of a given species may or may not be attractive to other of the same species. And if they don´t fall in love for each other, there´s no spawning, bring a new partner, please.

Ref. boredpanda