any feeder fish can easily introduce parasites and diseases. oscars should not be fed live food at all for this very reason. in addition, live food can make them more aggressive. this could cause problems for 2 oscars in a 55 (which may not work in the first place). many people consider a 75 to be minimum for a single oscar. i think a 55 is fine if done right, and 2 is really pushing it. TONS of filtration is needed for 2 oscars in a 55. what types of HOBs exactly? i have 2 whisper 60s on my 55 that has one foot long jaguar cichlid (very similar to an oscar). they are doing a great job. if you decide to do 2 oscars in a 55, you should probably also consider a canister to supplement the other filters, whichever type they are. you cant overfilter, but it is very easy to underfilter, especially with the fish and tank size ou are attempting. i only feed pellets, if they will take frozen that is good too. a variety of high quality pellets is the best.