Looks like Hoot beat me to the punch while I was wrestling with Photopoint, trying to get a relay pic uploaded to post. Anyway, here is the wordy stuff.
There is a diagram on the back of the relays package to help you wire it. What you want is a relay with a 12 volt coil and hook it up to the yellow and black wires from one of your PSUs molex connectors. The other part of the relay is the switch. For a water pump, get a relay with contacts rated at 110 or 220 volts (depending on where you live) and to be safe, rated at twice the amperage of your pump. If you use a relay for your pump, be sure to install a bypass switch so you can run the pump without the rest of your computer for filling and bleeding. You can swipe a switch and a 3 prong AC socket out of an old computer PSU.
The same rules apply for a pelt’s PSU. You can switch the pelt’s PSU on with a relay or if you are using a big external PSU, you may want to leave it on and use a relay to switch the DC on for the pelt. The Shack sells an automotive relay with DC contacts rated at 35 amps.
Be sure to heat shrink every connection. You don’t want to get zapped by AC or high current DC from a pelt PSU.