How large is the Dempsey? I have Dempsey's and as long as they are with fish of similar size, they pretty much keep to them selves. They usually become the dominate fish in the tank. Again, depending on tank mates. If your Dempsey is around the same size as your silver dollar and barb, the Dempsey should leave them alone. If you want to add mates to the aquarium, I would suggest anything with a little attitude. I have Red Devils, Urothalmus, Jags, Loiselli, Carpintis, Convicts, Salvini, Flowerhorn, Green Terror, Xiloaensis, and Hogaboomarums with my Dempseys. All of these fish can have an attitude. Some more than others. But I feel that if the aggression is spread out, and the fish grow up together, they tend to tolerate each other. And it makes adding others a bit easier. The only problem I have when adding a fish is if the dominant fish in the tank feel threatened they will attack the new addition. So I do have fights at times but it is usually against fish of similar size. My pecking order is my Hoga male & female, then my Red Devil male, then the Loiselli, Dempsey, female Devils, Salvini, Flowerhorn and Carpintis are even, Xilo, Uros, Cons, etc....... My jag is still small so it holds its own.