Can't believe I've been running this for a year and never thought to turn off the skimmer and see if it's the problem:biggrin:
It's a Hydor 600

It's super tight in my sump so Im not sure if this set up will be possible, Ill try it out though,
thanks

You can fix pump cavitation by throttling back the outflow of your return pump. This is what I have to do on my 120G with my reeflow. Just make sure you close off the outflow and not the intake. If this works you can either leave it as is or upsize your return plumbing.

Dont think my mag 9 has a restrictor on it. I do however, have a gate valve on the return line. Should I close that a quater turn?

yes closing the return line would work via the gate valve. This could also do 1 of 2 things, slow flow through the sump, thus allowing micro bubbles more time to settle out, and decrease the likely hood of the cavitation. There for you would be effectively eliminating 2 possibilities. Which is not always the best if your looking to get to the route of the problem.

But on the upside, i think you have enough info at this point to figure it out. Let us know how it works out im interested myself.