you can have a sump and not off gas co2... my set up is great... 125 gal with 4bps and my drop cheker is green/yellow
sumps just needs to be set up in a different way so that the typical large cascading water down an overflow and down into the sump is minimal
do you have a way to check your co2 levels in your tank? how are you putting co2 in the tank pressurized or diy?
adding mono potassium phosphate will help with your phosphate levels
also which kind of green algae? dust or spot?
how many hours are you running your lights?