You can do a number of things.
(1) Get an aquarium skrubber and skrub the inside of your tank.
(2) Do a few water changes a week.
(3) Do a good gravel vacume.
(4) If you feed twice a day change it to once.
(5) Get some live plants.
(6) Keep the light on for a maximum of 12 hours a day. (nothing more, if anything less then that.)

!!!Hope This Helps!!! Good Luck!!!