Silver dollars are tough fish but your 55 will be looking very cramped when they grow. I kept 3 of the redhook SD's in a 90 gallon and felt it was too small, the way they swim doesn't lend itself well to small spaces. Most here will tell you a 6' tank is necessary, and having kept them in smaller, I'd agree. Could perhaps get some small ones and replace when they get larger, wouldn't be a good long term solution though.