Another thing to consider is tank shape rather than gallons. The tank I have is about 47"X20"X31"(water level). Less gravel than a 24" tank would have.
The recommendation of 4 Oscars plus other fish in a tank that size is not a good recommendation. 4 keyhole cichlids would be a good recommendation for at tank the size of the OP's.
Am I the only one here that sees this?
Why do people have to cram these big, active, intelligent fish into these size tanks?
Even if you keep the fish fin perfect and disease free, is this really good practice?
So many modest sized fish In the hobby to choose from, let's put 4, 12+inch fish in a four foot tank. Sorry, but seems unfair and unnecessary to me. I don't think a solo oscar can be happy in a four foot tank.
Each to their own though.
I also believe silver dollars should be in much bigger groups to feel comfortable and in a tank of minimum, 6+feet.