According to my knowledge, a 55 gallon is too small for an Oscar
long term as opposed to popular belief. Several things I've read stated that they require at least 75 gallons, grow fairly quickly.....and you wouldn't be able to keep it with anything else other then perhaps a large Pleco.
However, a Jack Dempsey
would work in a 55...much like an Oscar
, but slightly smaller. My dad kept one once, very neat fish for sure. Though I don't think you could put much with it either, also like an Oscar
If your looking for a community, then both the Oscar
and Jack are Cichlids you wanna stay away from, that much I know for sure. How large is semi-large to you? Do you plan on planting the tank? Whats the Ph of your tap? Are there any specific fish you have your heart set on?