pexplumbing
I've noticed that in a properly (I assume) crimped Pex connection the tubing will rotate around the fitting it is crimped to – so that you have, in effect, a union.
So, I'm wondering:
You need to evaluate where the leak is really coming from. Use some paper towels to thoroughly to dry the whole fitting area. Then watch to see where water first appears. I suspect that you may find the leak coming from here:
..as opposed to the PEX joint. It appears that you did not use a Teflon sealing tape on the pipe threads.
Assuming you have or can get the clear tubes to attach and not leak, I'd suggest a 1-1/4" (or reducer and 1/2") PVC pipe parallel to your clear pipes, running above water level, and NO penetration for the CO2 feed line; not only run it in the bottom of the pipe as suggested by @bib, but run it inside regular, normally solvent-welded PVC vertically as well, all the way above waterline. No funky or fragile seals between plastics that don't solvent-weld nicely.
From fishtank experience I'd also suggest that you be open to running 10 tubes so that you can balance the airflow if it does not opt to cooperate and divide evenly in practice (one of those things where things are the same in theory, but often different in practice).
Best Answer
Should PEX connections rotate?
Yes, this is normal, and true of both crimp and cinch style connectors. It's also a useful property of PEX during installation, as it makes it even easier to dry-fit and pre-assemble sections and then fit them into place.
Can the connection be used as a union?
A union is designed to come apart, and in copper or PVC plumbing are typically used in two situations:
In the first case, a simple PEX connection obviously doesn't satisfy the need since it doesn't come apart, and so you'll need an actual union.
In the second case, the flexibility of PEX and fact that the connections can rotate does allow installations that aren't possible with copper or PVC. I've installed dozens of water treatment systems (typically involving complex turns and tight spaces) and cannot think of a situation where an actual union was needed because of the install. But if this is what you mean, then I'd say yes, you can avoid using a union and just rotate the PEX.
Rotary union
If you are trying to do something that involves regular rotation, where you'd normally use a rotary union, I'd say no -- you should just use a rotary union. The fittings aren't designed with rotation in mind, it's just a property of the style of connection. I'd expect if used this way, it's going to wear out the pipe and/or connector itself and the connection will eventually fail.