I have a MJ600 in a 12g cube as well, without issue.
I have the return split into two nozzles, to create more turbulence, but I still don't think that would make a difference.
Sometimes clowns like to 'powerhead surf'. I had a Saddleback that would do it every night for hours. The stupid fish was inches from the PH, swimming almost as hard as it could, for no apparent reason.