Yes, it could very well be because you have too many fish. That's a lot for a 75g.
For now, I'd suggest keeping up with daily WCs. Get more filteration on that tank as soon as you can...
We need to know the sizes of the fish and what you are using for filters too, so I/we can judge just how much bioload your filters are trying to handle. Right now, even if your fish are juvies, I can't imagine it not being a very heavy bioload.