You can use the aquastream.
The included power cable connector will give you the possibility to start your WC before you start your hardware. (just remember not to plug on any other hardware to your Power Unit before you have tested the WC.)
If you are using the Aquaero and a part called "Durchflusssensor" you will be able to get an alarm if the wather presure is too low. (-> pump is out of order)
P.S.
If you like to know a cheaper version, you can use a tempreture alert.
So Shoggy is right. The Aquastream- version would only provide a start as a kind of test after filling the WC. It is not really a good idea to do that every time you're starting your computer.
If you use the aquaero / flow sensor combination, you can configure the aquaero to switch off the computer immediately if the flow should stops. This is the safest solution.
Let's say you have a aquaero and the flow sensor. If you start the PC, and the pump would not run or the flow is too low, the system would run less than 5 seconds and shuts off. These 5 seconds without any waterflow is absolutley no problem for the hardware - you will have more cooling power than a cooler with a working fan