We will need to know some info on your tank in order to possibly suggest something. Tank size, all fish in the tank, live plants or not, water parameters (pH, hardness if you know it, temperature), how long has the tank been set up.
In the meantime, the silver dollar is shoaling fish, which means they live together in large groups in the wild. They must be in a group in the aquarium or they will not be healthy no matter what else. Fish that are evolved to live in groups can be severely stressed on their own, and some species also have social interactions within the group. Given the size this fish grows, it needs a largish tank. It is also vegetarian (eats plants). We have fish profiles here, second tab from the left in the blue bar at the top of the screen, or you can click on the shaded name of a fish to see its profile; information there mentions all this plus more.
By the way, welcome to Tropical Fish Keeping forum, as I see you very recently joined us.