Need some advice on aquarium cleaning.....?

So im over my cousin house and he has a 55 gallon tank. Its full of particles and debris. The filters wont filter it out. Should I take out 5 ponds of gravel and set aside, empty the tank and wash the gravel and start fresh. Put the "cycled 5 pounds of gravel" back in and add some seachem stability to build back up the bacteria. I have a sponge filter from my tank I can put in his to help colonize the tank with bacteria aswell. Its a freshwater tank and its so much debris and particles that his fish are "flashing" on the rocks and stuff from being irritated. (Im sure its cause of that) He hasnt tried the chemicals to "clear the tank of debris" would that be better cause his fish are suffering...I have never had this type of problem in my tank so im kind of lost what to do. He has 2 filters on the tank and it wont filter out the particles.