I use PEX plumbing for some secondary systems — ie 1/2” PEX lines for flow to a refugium, between my RO reservoir and SW reservoir, SW to sump, and a few others. I only use plastic push-to-connect fittings from Watts. No tools needed other than a cutter. Super easy and flexible to work with. No leaching concerns.
As long as you properly install them (square cut the end, push all the way into the fitting), they’re rock solid and I have had zero leaks. Install is also faster and easier than PVC.
I don’t see any reason you couldn’t scale this up to larger tubing to support higher flow systems like main drains and returns, but with a couple minor and manageable concerns: 1) larger PEX fittings may get pretty expensive compared to PVC; 2) more of the standards in our hobby are designed around PVC so things like pre-drilled bulkhead holes may not natively work (although you can probably solve that with just a PEX to NPT adapter); and 3) PEX tubing most commonly comes in rolls that can be a bit challenging to straighten. Straight pipes are available but you may have to go to a plumbing specialty store rather than just run to the local hardware store.
