Ammonia is extremely toxic to fish. It can kill them. I don't know how your other fish are still surviving. It seems strange that you have some ammonia, but zero nitrite and high nitrates and yet your fish are alive. That's A LOT of nitrates. I've seen fish endure 80 ppm
, but it's best to first introduce hardy fish at 30 or 40 ppm
. You'll need to do power water changes. That's a huge amount of fish and their bio load can crashthe tank and send it in another cycle which I think it has already. I suggest you research before you add anything to your tank. I'd strong recommend not adding anymore fish and possibly taking some out now rather than later.