Get a 12V DPST (double pole single throw) relay, not a SPST (single pole single throw) because it is safer to switch both 120VAC leads going to the pump. You wire the relay coil to one of your unused power supply leads using the 12V (yellow) lead and the black lead next to it. When the system +12V comes on, it keys the relay which allows the 120VAC to pass through it.
With the right relay, you can use a fan header for the 12V source also. A good choice is the Radio Shack 275-249, though it is DPDT, you just don't use the NC (normally closed) contacts.
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.