Maybe I was wrong, but I read it as him asking how to breed different types of fish as in he wants to breed clownfish, then breed dottybacks, etc, with their own species. As in, he wants to breed within species, but more than one type of species %% Sort of like setting up his own little fish hatchery %%
You can't cross species, as in you can't cross a mandarin and a clownfish, just isn't going to happen.