Rectorseal 5 is available at all the big box and hardware stores in my area. It's basically the consistency of toothpaste...it hardens up a little over time, but it's not like solvent or anything like that that will permanently bind the two pieces together. You can disassemble without any issues. It's just pliable/soft enough to fill voids but viscous enough to prevent water from leaking through the threads.
Interesting that Aqua UV thinks it might clog the valve to the filter...I guess if you are really heavy handed with it that might happen, but just to be safe I would definitely follow what Kendragon is saying and stick with teflon tape (at least for the union fitting to the filter).
Here's the FAQ from Reeflo's site:
SETUP AND INSTALLATION
ATTACHING PIPING
· Avoid Teflon tape on the threads-the threads are tight, the pump is not high pressure-Use liquid sealant around the top threads
· Avoid overtightening-Not a high pressure pump-Hand tighten until snug and STOP!