Silver dollars get fairly large, about 6" if memory serves, and require small schools of 4-5 fish in order to thrive. I still maintain, however (as someone who has kept oscars for over 25 years) that 2 oscars in a 90g, with double capacity filtration, should be fine. I have 3 oscars, as well as 3 red-eared sliders, bichirs, and eclipse cats in a 220g water garden. I do a 30% water change weekly, clean the substrate every other water change, and have never had nitrates over 40 ppm. To be fair, the oscars aren't yet fully grown at about 7-8" each, but the turtles alone probably have double the bio-load of all the fish together. But, your pet's health and welfare should be your first priority, and many of the people in the forums live and breathe fish, so their advice is not to be taken e grano salis. I love my fish, but have too many other irons in the fire to study as much as I should.
