Five potentially ten-inch fish is potentially a whole lot of fish. Even if yours stay the more likely five to six inches, they are going to need some room to swim around. Third, Silver Dollars do not grow nearly as fast as your Oscar. Make sure that when you buy them they are big enough to outpace the growth of your Oscar’s mouth. I’ll end Hello fellow aquarist, I have currently 3 4 inch Blood Parrots and 5 1 inch silver dollars in a 25 gallon tank and I want to upgrade because I heard that they grow big and the silver dollars grow fast. Does anyone have an Indea what tank size I should get (maximum 125 gallons please) Thanks! Silver Dollars are a lively schooling fish and best kept in groups of three or more. Rocks, plants, and driftwood help mirror its natural habitat and will help to reduce stress on the fish, though plastic plants may be necessary due to their herbivorous nature. They do best in soft, slightly acidic water with high filtration.