Lets start at the beginning. First look at your bulk heads insides. Are there any threads on the inside of either end? If not then you have what are referred to as slip fittings. They are called this because your solid PVC pipe fittings or PVC fittings slip inside the fitting (bulk head) and are solvent welded into place. If they are threaded there are threaded fittings made to use with them. Secondly, remember the bulkheads are supposed to be put in hand tight, do not use wrenchs. Third you should put the silicone plumbers grease on the bulkhead gaskets, put them on the bulkheads and install yhe bulkhead in from the inside of the tank. The bulkhead nut goes on the outside of the tank without gasket or lubricant. Silicone tape can be used on the threads of the bulkhead where the nut screws on , but it is not necessary. I personally use PVC pipe for all my drain and return lines, and oversixe all plumbing lines and valves. Less restriction, less friction equals greater flow. Remember to use as little vertical pipe as possible in your drain lines as they are restrictive , cause turbulence and are noisier than vertical or angled pipe. Use two 45 degree fiitings instead of 90 degree fittings where possible in return lines. Bulkheads are expensive unless you look around. I pay $3.50 for 3/4" and $4.25 for 1" from a local company that makes plastic water tanks for hauling and storing drinking water. The local hardware stores want double that, and the LFS triple that price. It would take two too three full written pages to take you all the way through plumbing an overflow and a good return system with unions and ball valves, over flows, return line, anti siphon drain holes, Durso Standpipes, selection of pump size etc. That is if you are a novice at plumbing and have not set up a tank before. I would be glad to write you through it, but not on open forum. If you need complete information or a lot of information send me a personal email for information. One word of caution though, if you think vinyl pipe is easier, remember easier is not necessarily better. Vinyl tubing has very restrivtive fittings. Think of it this way, a 3/4" diameter PVC pipe has an interior diameter of 3/4", whereas a vinyl pipe that is 3/4" in diameter needs a fitting that goes inside the pipe greatly reducing the inside diameter of flow at every fitting. PVC pipe has its fittings around the outside of the pipe therefore the flow diameter is not reduced at fittings. PVC pipe maybe requires more patience in its installation, but the outcome is much better than with vinyl pipe.
