two months is enough time to force cycle a tank with fish inclusion. since the tank is fairly good size, the animals just rode out the low ammonia levels until natural mechanisms allowed for enough bacteria...if your tests at the lfs showed zero ammonia this is not uncommon though the mass mode is to complete the cycle first. either way, your cloud is most likely from sandbed disturbance, most people will not rinse a sandbed to zero clouding ability before use, so it clouds with the first water changes.
its also secondarily possible to have reached the ammonia processing max for the new system via fish waste and feed accumulation. that lowered ability to oxidize waste, combined with any floc from the bed is the most likely combo. if you wait it will go away, but also continue to happen for a while.
if that was my tank id hold the fish in buckets, take down the tank, rinse the whole sandbed to pure nonclouding, put back, 100% water change, reacclimate fish. it would then behave normally at that point. larger tank owners typically wont reset like that, so small micro filtration can help, and buy at least a salifert ammonia test don't buy api
