| They COULD be bad if they collect on corals.
Now you say they go away when you turn off the return pump. I'd inspect the return pump itself, make sure a seal isn't loose. Also take a look at all the tubing coming from the return pump... there could be a spot where its not sealed completely and might be sucking in just a tiny bit of air.
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90g Mixed Reef | 30g Sump/Fuge | 2x 250w MH 12k | 2x T5 Super Actinic | Euro-Reef CS6-1 | ReefKeeper 2 | 
