All filtration connections in our pools are made in 50mm and 40mm PVC Class C Pressure Pipe. At the left is an illustration* of a PVC part called a "MAC Union"

This useful part allows the two connecting pipes to be easily disconnected without having to physically cut the pipe in two

MAC unions are used extensively in filtration connections, as it eliminates cutting through pipes over the years as parts need removasl for replacing. As they are PVC plastic, tools MUST NOT used to tighten them. Your filtration system is "mobile" as water temperatures change during the day causing contraction and expansion and loosening fittings, so this will occur from time-to-time.

Because of this process, they will "work loose" many times over their expected lifetimes, and will require some input from you - the poolowner - to give them a little assistance from time-to-time.

Simply grasp the knurled outer ring (top red arrow shows this) and rotate it in a clockwise direction to tighten it as shown (looking at it from the left side)