I recently setup a new 90 gallon system recently and had the same issue. I thought maybe it was my diamond goby causing it. He did cause some but only the first day or so. That guy is an awesome sand cleaner. Anyways, the cloudiness issue was driving me nuts. I changed the carbon, nothing. Added filter sock, nothing. Water change, nothing. Than as I was reading around, I determined I'll give an UV sterilizer a shot. I went to my LFS and purchased a DeepBlue submersible uv sterilizer (I actually sent my wife). Threw it in my sump. By that night it was already more clear. By the next day it was insane how clear my tank was. I've only had this in my tank for 2 months but so far I'm a huge UV fan. Some say it's not good on corals. My sps and lps corals are actually doing better. Later on I'll probably get a better unit but for now this DeepBlue one is great. Okay, that was definitely longer than it needed to be. Good luck, keep us updated.
I forgot to add...
I most likely had some free floating algae and/or bacteria therefore the UV took care of that. If you ruled out a possible calcium precipitation than that's most likely the issue.