Powerhead spitting micro bubbles

[66chuck]

How does a completely submerged powerhead w/out an airline spit micro bubbles? Mine spits out bubbles every 5 minutes or so. Didn't used to, any ideas as to why or what to do to stop it?
Thanks.

 

[f14peter]

The reason is probably due to "Cavitation" which is air bubbles formed by something moving quickly through water (Big problem for submarines that want to remain as silent as possible as their propellers can cause cavitation. Untold millions has been spent on designing cavitation resistant props).
Just a WAG, but since it just started, may want to take the PH apart and see if there's any crud built up on the impellers/blades that are roughing up the flow.