Your two main concerns for using 3/4" poly underwater. 1. It will take close to 3/4lb of weight per foot of airline; 5/8" ID requires 0.5lb/ft, thus 100ft weighs 50lbs. . 2. Don't put extra weight on the airline just enough to hold it down because you should be lifting the diffuser out each year and cleaning it. Pulsing the air to the diffuser often does not adequately clean it for good efficient bubbling. Membrane diffusers should have proper maintenance which means a scrubbing each year usually at the beginning of the aeration season - spring.