This is due to algae in the water. A few things can cause excess algae. The first thing is direct sunlight on the tank. Make sure your aquarium is positioned where this can’t occur. The next thing is excess nutrients. We suggest you have one inch of fish per one gallon of water. For example, a 10-gallon tank should have only 10 fish that are one inch each. More than this will elevate ammonia and nitrite levels. As a result, the algae will have plenty to feed on. To solve the problem of green water, do a 50% water change. Scrub down the insides of the glass with a magnet scrubber or scraper (but please…no soap!). Remove any artificial plants and ornaments, and pour boiling water over them. Wait one whole day, then do a 25% water change. Cut back on your feeding. Your fish should eat all the food in two minutes or less. This should help get your tank in good shape. You may also install a UV clarifier. This is installed in-line with a canister type filter that is placed in a cabinet or stand beneath the aquarium.