Aquarists tend to use PTFE (Teflon, aka PTE) tape a lot when doing plumbing using threaded fittings such as unions, bulkheads, and other types of plastic fittings. The big reefkeeping forums as well as local club forums are full of advice about how many wraps of tape to use, which type and brand of tape is better, etc. Tape is easy to use, and is not messy like PTFE pastes and other semi-liquid and paste sealants. Consider this, however: Teflon-based tapes and pastes are primarily intended for use on metal threads. When used on plastic threads, the tape or paste acts as a lubricant, which leads to easy over-tightening and causing excessive stress on the female fittings, which in turn all too often causes cracking and splitting. Too many wraps with tape only makes this more likely to happen. Some manufactures of PVC fittings actually say DO NOT use any teflon-bearing products w/ PVC fittings. Others say it's OK, but with cautions against over-tightening PVC fittings. Instead of Teflon-based tapes or pastes, many manufacturers recommend the use of a plastic-safe non-setting or soft-setting pipe thread sealant or joint compound.

This article from a large PVC piping manufacturer makes for interesting reading: http://www.lascofittings.com/threads
Here is another link from another manufacturer: http://www.spears.com/how_to/FG-3B-0...06_English.pdf

I personally have used Rectorseal 5 for years on aquarium plumbing and also my home's sprinkler system with great results. In one aquarium build, for example, I simply could not get the five 1.5" threaded bulkheads in the bottom of the tank sealed properly after many attempts... I used PTFE tape with varying numbers of wraps, thicker tape meant for natural gas use, Teflon pastes, and combinations thereof - nothing worked. I then tried Rectorseal 5, and it worked superbly! For many years now, it has been my "go to" sealant. Rectorseal 5 is widely available at many home improvement and hardware stores. In my experience, here is how to use Rectorseal 5: apply the sealant only to the first 3-5 threads of the female fitting; fill those first few threads with the sealant, but do not use an excessive amount. As you thread the male part into the female part, a bead of sealant will form and be pushed along by the rim of the male fitting. If you fill too many threads or glop it on too heavily, the bead will be too big and may drip or run, or cause obstruction. Tighten the fitting snugly, but not too tight. Plumbing up to 2" assembled with Rectorseal 5 may be used immediately, but I prefer to let it sit overnight before exposing to water.

Here are pics of a 1" flow sensor & union and a 2" flow senor & union assembled with proper beads of Rectorseal 5 - not too much, not too little:

FS100-sealed.pngFS-200-sealed.png

Some other suitable pipe thread sealant products are:

Spears 75 Blue
Oatey Megaloc
Oatey Great Blue

Please be aware that Oatey Gray pipe joint compound is NOT plastic-safe. Some Rectorseal brand products also are not safe for plastics. If you look at other similar products, be sure to check the label carefully - if it doesn't specifically say that is for use with plastic pipe fittings, assume that it is not safe.

This is based on my personal research, experience, and opinions; your experiences and opinions may understandably differ. Regardless of what form of sealant you choose to use, be sure to verify that it is plastic-safe & follow the instructions. And do not over-tighten.