I don't have the biowheel filters but from what I recall, I think those wheels are suppose to spin in order for the aerobic bacteria to get oxygen to remove ammonia and nitrites. I've read some reviews too that these filters aren't the best because they lack the surface area to host BB. I honestly don't know how effective they are. I personally use Aqua Clear and a Fluval C series. You should research how to resolve your non-spinning wheel issue because I suspect it's not doing its job at filtering your water properly and you're being under-filtered right now. Either that, or replace the whole filter and add in extra bio media (i.e. BioMax).
As for buying a used aquarium, did you take any precautionary measures of sterilizing it before use? By not doing so, you may have inadvertently adopted more than what you bargained for. How about the new rocks you introduced to your tank?
As for your other fish getting popeye coupled with your two gouramis (the latest having dropsy), that is a red flag to me saying that there's some serious poor water quality issues. Both fish ailments are associated to weakened immune systems due to poor water quality. Fish are susceptible to bacterial and other fish diseases when that's the case. I would seriously consider upping your PWC's, gravel vacs, and resolving your filtration issues moving forward.