Ok, What about a ghost knife or something like a silver sharkYour not going to like this but your at max capacity with an Oscar. At a yr your looking at roughly an 8-10" fish then with another few yrs a fat 12" or more oscar. Its a lot of fish for a 75 there's not much room for anything not considered food.
You could go with something expendable for awhile. A lot of people do dithers until they're fish gets big enough to not be shy
Thank you for you help