Working with pvc

[JimE]

Can someone explain to me the difference between "standard threaded bulkhead" and "double threaded bulkhead"?

That's a bit of a gray area, depends on the suppliers terminology. Double threaded should just have female threads on both the "inside and outside of the tank". Std would only have a female threaded connection "outside thetank" and just a smooth bore inside.

what they call spa flex, clear with diamond shaped mesh

No, spa flex is solid white with a spiral wire (plastic). The proper cement (in the common weld-on brand) is #795. This is good for flex-ridgid and IME I haven't had any problems with ridgid-ridgid in low pressure applications. On a side note, most spaflex will also take and inside barb fitting and ss clamp.

except i was told to use the same glue as you do for regular PVC with it by the people who use it all the time (hot tub builders)

"the same glue" could be any number of products. There is a weld-on 747 Pool 'R Spa that is rated for both, but it is usually hard to find and is more expensive than 795. What I would call regular PVC cement is 704 or 705 and it is not suitable for flex. The flex glue is more flexible because of course, flex flexes..... As far as advice from hot tub builders, think brain cells vs glue and resin fumes.....

Jim