Yeah I've seen a bunch of those too Wes but you can also find sites and stores that will tell you a Hippo Tang is fine in a 24 gallon biocube. That the Freedom Filter is fine to filter and skim a tank up to 200 gallons, I could go on.
My issue here is that none of those sites that recommend gluing spa flex manufacture the stuff. Also suspect is that they say the word GLUE. PVC is Solved together not Glued. Glue is for construction paper and elbow macaroni. I would think that a place expert on plumbing would know the difference.
I live in an area that was mostly blue color people for 50 or so years. Every professional I bring into my home says:
"The thing with this area is that it was all cops and firefighters, every cop and firefighter thinks that they're a plumber, a contractor, a tile guy etc. They then take me around the house and show me everything that was done wrong. "Yup, here's a Marine Park special right here...." I've got a million things in my house that look ok but are not to code or are rusting because of a little thing that I would never have thought of. A master plumber knows not to use a black pipe cap on copper pipe but you or I might not know that. The guy at my hardware store doesn't' know that.
I too have used spa-flex solved to regular PVC and it was fine, it even held tight. I'm just pointing out that the manufacturer of spa-flex says that it should never be glued and it wasn't meant to be glued. I'm sure they know more about their product than the guy selling it with "spa glue". I'm also pointing out that I asked a few people qualified to answer such a question. A Licensed Master Plumber who has installed thousands of these in condos all over the place. He also said "No Glue, never glue on flex pipe"
When you use PVC, or any pipe you have to secure the pipe down so it doesn't and can't move. Really important! Moving can weaken and stress the joints causing them to fail. Rigid pipe won't move very much but flex on the other hand is designed to move around. That is why you shouldn't use glue. Even "spa glue" or "thick glue" or "high solids content glue" isn't appropriate. If you look around your house at any pipe none of it is held by thick glue. The only place you'll find thick glue is on your gas lines but those are held in by threaded fittings, are cast iron pipe and are going to be secured everywhere.
Lastly, all I'm saying is if our tanks were inspected by NYC inspectors I don't think they'd be up to code due to the glued spa to PVC joints.