just an update on the skimmer water level. i added rock and more sand which raised the water level in the sump to over 15".(letting evaporation work and raise the salinity a little) the skimmer works great at that level. after seeing it work in 15 1/2" water level i think you can put it in water all the way up to neck. maybe even right up to the neck of the collection cup, where it twist on. and to think i was worried about 12".
no issues with sucking in air at 15 1/2".