ok you need to get rid of that airPiump whatever the fuck it is with your circulator pump... you are wasting gpm. that air shit is good for a recirc pump for the res or whatever but is killing your overall watering.
larger return tubes. i cannot understand why 1/2" would not work for you. i think you may have gravity working against you. try putting your buckets up higher so they will use gravity to return feed. even an inch or two will help.