20 gallon fish tank keeps green water?

There are only two factors involved in growing algae. Light and nutrition. The commonest cause of free floating algae is an abundance of nutrients caused by either overfeeding (feedings should be small and once daily) or failure to keep the water clean (high levels of nitrates and phosphates, nutrients). Every time you tear down your tank and "clean" it you kill the beneficial bacteria that will eventually bring balance and harmony to your water conditions. I believe that your tank maintenance methods need improving, water changes should be only 20% at a time and accompanied by careful "vacuuming" of the substrate. I'd suggest that you purchase a book that describes proper aquarium maintenance and study it well. You need more information than you will get on a forum such as this.