75 gallon water will not clear up

[btrecordingz]

Hi everybody. Just joined up cuz Im losing my mind over my 75 gallon tank. Ive had the tank for about 9 months now and all of a sudden my water just became disgustingly cloudy. I havent done a thing to the tank except a 15% water change every Wednesday. The fish arent stressed and the parameters of the water are in check, its just horrible to look at.
What the hell happened?
Filters- Aqua-Tech 30-60 HOB
Penn Plax Cascade 1000 with bio rings, bio sponge, and filter floss
Penn Plax Cascade 700*as a water polisher.

My water was shimmering pretty much since after it cycled about 8 months ago...and now its just nasty. Any ideas?

 

[BlackShark11k]

usually that's caused by a bacterial bloom. are you sure you didn't move anything around???

its odd that your water parameters test ok. hopefully someone else who knows whats going on will chime in.