Bubbles in Closed Loop

[sphelps]

It's also possible that the intake plumbing has become more restricted resulting in a lower pressure at the pump intake, if the pressure is low enough it can actually liberate dissolved oxygen. To test for this just throttle back the pump output halfway, if the bubbles stop this is likely the issue. I've seen this happen with these pumps when people don't plumb large enough pipes and bulkheads. Untamed is also correct regarding cavitation which is essentially the same thing, a good cleaning could fix things.

If not it could also be the seal on the pump, anywhere you notice salt creep could be a potential leak.