Even with bi-daily full water changes my fish tank keeps turning green, What am I doing wrong?

I have a 60 gallon tank with about 8 small fish in it. Two of them are algae eaters. (Im not sure of technical terms). The others are tetras and rather small. I have already thinned the tank out by removing 5 very large red belly pacu's. I thought they were the problem and I have proven myself wrong. We are still having to do a 90% water change as well as cleaning all the rocks and decorations with the the water change every other day. I do use a ph kit as well as formulas for making tap water safe and an algae deter ant formula. We are changing the filters whether needed or not with each water change. So what am I doing wrong. We feed only whats necessary and have even skipped two days to see if over feeding was the problem and it has not helped. My husband thinks its a tap water problem and I am wandering if its our pump. So I need advice quick, I'm close to giving up. Someone help please!!!!!!!!!!!!!!!!!!!!!!