Closed loop causing many micro bubbles

[Scales]

I have had problems with cavitation on all the mag pumps on my systems (2 - 12's and an 18). Input flow restraint was remedied by placing the two 12's in the sumps. Even so, I still have problems with one mag 12 which must be throttled back (output) a bit to prevent micro-bubbles. At full open, it sounds like "rattling screws" and spits out bubbles. I have tested for leaks and can find no evidence of any (the pump itself and the housing connections are submerged). The mag 18 isn't causing problems currently...it is on a closed loop with a 1 1/4" input reduced right before the housing to prevent input restriction...the output is theoretically restricted from the initial 1" to approxiamtely 1/2 (it used to go through a squid which was removed after it failed to "switch").