no offence but your Oscar is not a moron. I think it's you who is the moron. For one your tank is tooooo small to have a Oscar and "other cichlids" like you say. A 55 is barely enough for one Oscar all by themselves. Your filter by the sounds of it is probably some crappy whisper or something. Go out and buy a real filter like a Aqua Clear 110 which is like $50. If you want to spend even more look into a canister filter.
Don't change the filter cartridges...... you need that bacteria to filter your tank. Go to google and type in the words nitrogen cycle. Read this and you will then understand why you don't just replace cartridges.
Once again your Oscar is not a moron but you need to learn on how to care for him. Your Oscar is just suffering because the tank he is in is overcrowded and there is no biological filter, which in turns causing the tank to have excessive amounts of ammonia and nitrite.

Picture living in a 10 foot by 10 foot room with 10 people with absolutly NO air circulation or filtering. Imagine what it would be like when everyone takes a crap in the corner. Thats basicly what your Oscar is living in. If that 10 by 10 room was a real thing I would ask what moron put them in there.