I have 2 sliders in a 75 gallon tank. They have lived in this tank all their lives, (about 20 years). It has recently developed a green alge that really muks up the water. I did a complete tank cleaning and within 3-4 days the water is green. I used alge control and right start when I refilled the tank.I use 2 penguin 660 powerheads( up from the 550) with an undergravel filter system. Can anyone suggest what I can do to solve this problem?