Unplug the PS from the wall. Unplug the PS from the motherboard (ATX connector). Then you can take a piece of wire, strip a bit off of both ends, and then push one end into the green wire on the PSU MB connector (there is only one green one), and the other into any black wire on the same connector. This will "jump" it so that it will turn on without being plugged into the MB. Plug the PSU back into the wall. Anything plugged into it will now have power.
Or, you can do like I and many other do and go get an el-cheapo, used, whatever PS from Ebay or a local computer store for like $5 and use it only to power the pump. If you get an older AT style PS, you don't have to jump the black and green on the PS-MB connector. Otherwise if you get a cheap ATX PS, then just jump the black and green, tape the wire in place, and only use it for leaktesting.
You can also buy ATX PSU testers that will ground this for you, but they usually are like $15-$20.