it sounds like your putting just enough back pressure on the pump to decrease preformance when you raise the water level in the skimmer body.
some pin/mesh wheel pumps like a bit of back pressure, some dont. i think it has to do with the design of the volute/venturi and the pump power
vs. the volume of the reaction chamber.
i wouldnt think a laguna/askoll pump like that would b so easily effected, im suprised.
the other thing is, maybe there arent enough holes in your bubble plate?? i'll look to see if you posted a pic.
and you probably dont need to be feeding a mj-1200 at full throttle into the skimmer... that too might be effecting the amount of back pressure on the recirc pump, depending upon your skimmer outlet size.
also, after looking at the pics more, i think the inlet diameter to the recirc pump is too big...it should be the actual size of the pump inlet on the volute.
I don't think its back pressure as its a closed loop, there is no head involved, my "guess" is the bubbles being drawn in are more than the volute can handle, combined with the reduction in ability to draw air from depth of water, bit complicated but thats why recirc n/w probably are not ideal for this where the air stones should really shine.
"also, after looking at the pics more, i think the inlet diameter to the recirc pump is too big...it should be the actual size of the pump inlet on the volute."
the actual intake side of the pump is stock so that didn't change, you still need to create some draw for air intake... maybe a small reduction in diameter will increase draw? more experiments lol
"the other thing is, maybe there arent enough holes in your bubble plate?? i'll look to see if you posted a pic.
and you probably dont need to be feeding a mj-1200 at full throttle into the skimmer... that too might be effecting the amount of back pressure on the recirc pump, depending upon your skimmer outlet size."
no burping at all as Id think you'd get if not enough holes.
mj 1200 is 295 gph - head about 200? maybe, next go round will be an mj600 see what that does, with the enhanced dwell time I don't know if faster turnovers are good or bad