Is just a hang on the back filter good or bad on a small tank?

I have a tall 48 gallon tank with not much room underneath for a sump system. I do have a hang on the back filter on the back that I dont know if it is helping or hurting my tank. I feel like it is trapping waste and increasing my nitrates. Do I need to completely remove it or just leave the filters out of it and just let it circulate? I have a bad brown algae bloom and my two corals that I have are almost dead. My nitrates jumped sky high from one week to the next and I have done about 5 water changes over the past two weeks to try to get them to come down. Any advice?