zoathid`s dust

[snooper]

i`ve recently noticed some of zoo`s have closed up and look like they have dust on them,once they have this dusty looking stuff on them they don`t seem to open up again.these zoo`s are in different area`s of the tank so `im ruling out a flow problem as i have plenty of flow in the tank.i have 4 T5`s and a 150w 10,000k halide lighting, the water is testing spot and i use a turkey baster once a week to blow off any dust on the zoo`s.