slip or SLP is the smooth joints that you glue together with solvent. FPT is threaded. Female Pipe Thread. True Union means it has the screw together joint with the O-ring seal. The True Union joint can be taken apart and put back together. If you put a check valve in, your going to want to be able to take it out to clean it regularly.
Every bit of advice I've seen on plumbing an aquarium says not to use check valves. They are prone to fail, or at least leak because of the buildup in the pipes, and they restrict the flow.
I've never used one, so I'm only passing on what I've read. It came from the internet so it must be true