It will get cramped pretty fast. The flower horn will already be too big for the tank in the coming months and the common silver dollars get a good size. Unless you get faticus (however you spell the tiger ones) or I think thin bars are smaller too but don't quote me on that
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Silver dollars need room to stretch out. I have 5 in a 6 foot tank, and they use every bit of it. I wouldn't keep a shoal of them in a 55 myself, they'd be hitting the glass constantly IMO.