Welcome.
Your tank is not sustainable as is.
The fish you have could probably not be housed properly and long term in a 300gal tank. Just one oscar fgrequently is too much in a 75g, let alone with other cichlids.
The single red hook dollar is also not quite suitable for a 75g, and they do best in groups, which require a much larger tank.
I hope others offer you input, and suggest you consider what is/are a couple of smaller fish you like most, and stick to that. If you want a community, a 75g limits you to much smaller and less territorial fish.
A 75g is a very nice tank, in fact one of my favorite size tanks, but what you have in it is way, way too much.
Sorry for the bad news, and good luck.
PS. Your store is doing you a dis-service sellong you all those fish to be lept in a 75g.