It sounds like a bacterial bloom from rotting food. Two possibilities would be that you are overfeeding, or if you are feeding pellet food, the pellets are too large for the fish. Pellet food is compressed powdered fish food. If the pellets are too large the fish will suck on them and spit back out the excess. Purchase smaller pellets and/or cut back on the amount of food at each feeding. All the food should be consumed within 3 to 4 minutes. If you have flake food or pellets floating around uneaten after 5 minutes, you've over fed. Fill a clean 5 gallon bucket with water from the tank. Move the fish to the bucket. Remove the decorations. Turn off the powerheads and any other filters you have running. With your hand, stir the gravel in the bottom of the tank. Your tank will probably look like muddy water when you're done. Allow the excess food and detritus to settle. This will take about an hour. It will settle on top of the gravel. Using another bucket, use a hydrocleaner to siphon off the detritus and rotting food. Crimp the hose of the hydrocleaner to regulate the suction. Don't siphon more than 50% of the water. Basically you are doing a 50% partial water change. Fill the tank back up and treat for chlorine removal. If you don't want to use chemicals, you'll need to wait 24 hours before putting your fish back in the tank. If the fish are going to remain in the 5 gallon bucket for 24 hours, you'll need to hook up an air pump with an airstone and drop the airstone in the bucket. Or you could prepare for the partial ahead of time by prefilling buckets of water 24 hours ahead of time. After the fish are back home, turn on all filtration. The tank should clear in 3 or 4 days.