I've used pvc glue on drinking fountains in plants before, no problem.........I use it on all my tanks, no problem too. On the mag12's, I run a mag24 on my 175bow and I think it is not enough pump. Run a pressure rated pump, mag's are basically large powerheads, they're not really pressure rated. By the time you put them through 3 unions, a bulkhead, a ball valve, a check valve, 6 90's, they're flowing 20percent of what they need to be. I have a blueline on my 300gal that will draw a vortex from the outlet nozzles from the water shooting out of them so fast, the cooling fan is a little noisy on the pump, but man that thing rocks, my water is crystal clear. Iwaki is prettu good pressure wise, and a little quiter than the blueline. Dolphins are good if you buy the right ones. Don't buy any old one's the old one's had seal problems, they have since corrected the problem from what I hear. Remember also if you are going to use a big pump, you need to feed it with a large bulkhead like a 1.5'' or 2'' so the pump does not cavitate.
