Why is the water in my aquarium cloudy?

I have a 55 gal tank, I do regular maintenance on it. I do 40% water change and gravel cleaning every 3-4 weeks. I do not over-feed. Although, its hard to tell if I am over-feeding my pleco. I have " easy balance" I add every few weeks to keep the ph balanced and "aqua safe" to dechlorinate the water. I have a very nice external filter. RENA FILSTAR. A very expensive filter. Everything has bee great up until a week ago, the water became very cloudy. My filter was making a strange noise and I checked the flow by separating the output line where it enters the tank. The flow was very slow and at times no flow at all. The next day I did a 40% water change, did gravel vacuum, and disconnected the filter and cleaned it out. Hooked everything back up and it started flowing fine again. But it's been two days and the water is still cloudy.???? I have a total of 9 fish. 2 krebensis , 4 tiger barbs , 2 black fin tetra, and a pleco. 2 fiddler and a rainbow snail.