[Myka]

I only put a union on the return pump. If you put unions under the bulkheads, then if you ever move the tank you have to cut the unions off in order to get it to sit on a flat surface (like a truck bed) because you can't remove the bulkheads with the unions installed below them. If you want to replace the sump then you can just cut the drain lines and put them back using a coupler fitting. Neither of these situations tend to happen very often/if at all.

You don't need a ball valve on the return pump if - you chose a pump that fits the tank, there is only one return line, and you don't use a manifold. They are only $5 though, so if you're concerned, then go for it. If there are two return lines then the ball valves are handy to make the flow through each return even. If you do a manifold then you should put a ball valve on the return too. If the return pump is too big you will need the ball valve to dial back the flow so you don't get microbubbles in your display.

A manifold is a great idea if you have the extra flow from your return pump. If your return pump has barely the flow for the return then it won't power a manifold anyway. Have you picked your return pump yet? What are the dimensions of your tank? Do you have extra flow or not?

Try to use 45 degree elbows instead of 90 degree elbows as they reduce flow a bit less. I also recommend, if it's your first time plumbing or you just aren't that confident, make sure you have at least 2" of pipe between all fittings so that you have room to cut the pipe and re-glue differently if need be. If you glue all the fittings with no visible pipe between them it looks real good, but if you make one boo boo then you throw the hole kit and kaboodle out! Also, I made a 3-valve manifold one time, put it all together and one of the valve leaked in any position other than full open or full closed. I can to cut the manifold out and redo it. Luckily it was just SCH40 ball valves so it was only $15 and some time.

Use lots of glue. I like to use the clear HEAVY DUTY glue. I prefer to sand the ends of the pipe than to use the primer. If you use primer you can't dry fit the connections after you primer them as they will stick. They stick bad enough you can't get them out, but not good enough that they will hold water. That's a crappy lesson to learn. So I sand each end of pipe instead. Put glue on the pipe end, insert into the fitting all the way, give it a 1/4 turn, then use your finger to run around the joint to clean up the excess glue (like running your finger along a silicone joint). The 1/4 turn makes sure the glue is spread all over, but don't turn it a bunch (some people naively recommend this) as you will get air bubble in the glue. Air isn't water proof (lol). When I was a carpenter years ago, a real plumber (haha) taught me this 1/4 turn thing.