There's a lot of things that can cause water not to look pristine. Is it cloudy, or is it fully of waste, or is it algae? If it's tinged green, you have an algae issue. If it's full of waste/food, you need more filtration. If it's cloudy, you could be overstocked, may need to filter out smaller particulates, or you may be feeding a type of food that is bad for water clarity. You also may need to change more water than the 20% you're doing, or you may need to change it more often.
Basically, I guess I'm saying that without more information, all we can do is guess. What size tank do you have? What fish do you have in the tank? What do you feed them and how much and how often?