A 90 or a 125 isn't going to be suitable for all those fish, especially when they start growing and putting out more bio load. To give you an idea of where you're at, most on here would agree that a 125 is ok for a single full grown 12" oscar, that's it.
Go bigger, if you can, or arrange to lose some of your stock, maybe your LFS might take some fish off you for store credit.
If you can't go any bigger or lose any stock then you need to put a water change schedule in place that keeps them nitrates in check, and i'm telling you now you're going too be busy. Otherwise, in time, a floating oscar will be the least of your worries.