Silver Dollars are very fast and very skittish, need to be kept in groups of six or more. They do well with Oscars. I keep a Pterygoplichthys disjunctivus, can get to about 18". It's an armored catfish (pleco) that has armor plating rather than scales. My Oscars didn't like him one bit but they couldn't hurt him. He totally ignores everyone else in the tank. Is a real klutz when looking for food and knocks over decor and plants. Took all decor and plants out and has been fine without them. I keep a small piece of driftwood in there for him to try to push around.