Established 75 gallon aquarium turned cloudy?

It has 5 adult african cichlids, a large pleco, a pictus catfish, and a marble sleeper goby. the tank always was crystal clear then one day it turned cloudy. so i took about 30% of the water out and vacuumed the gravel out. its been about 3 days since and if anything it looks worse. I have the 75 gallon marineland bio wheel canopy system. so it has great filtration. please give me some advice! thanks