Just a for instance, something to ponder.
If your fish that are together now, and are not too crazy, they may have set up a hierarchy/pecking order that works for them. Maybe?
Sometime if you remove 1 or 2 zonatus(or any species from the situation, for that matter), the remaining individual can become hyper-dominant, and become aggressive to the other left over species, that it may not have, before.
Anytime to add or remove, you set up a new scenario, that may be worse, or better than the last.