Your filter is working, but it sounds like you are seriously over feeding. At feeding time you should only put enough food in the tank that the fish can consume ALL of it in 30 seconds of less. It should never have a chance to drift down to the filter intake or the bottom of the tank. Feed an average of 1 flake of food per inch of fish. So, for example, a 3" fish only needs three flakes at meal time.
For now, vacuum the gravel and take out one third of the water, replacing it with fresh, conditioned water. Test your water quality parameters for ammonia, nitrite and nitrate. They are probably high. Take the intake tube and filter cartridges out of the filter, and clean them. If the flow rate has slowed down, also remove the impeller from the filter box and clean it. Once everything is clean again, add Aquarium Bacteria to the tank. You need 0 ammonia, and 0 nitrite before you can add any more fish. Once you can add fish, add only one inch of fish per 10 gallons of water, and while the fish are floating in their bag(s), acclimating to the water temperature, add more bacteria to the tank.
Date published: 2018-11-16