I use them on my tanks - I am too lazy to build my own!
They come with reducer bushings to accomodate 3/4" mpt plumbing and these do need to be cemented into the heater module in and out flows. PVC cement works fine. If you aren't plumbing with PVC pipe, you should also check out the Hydor in line heaters. They have a 300 watt version - same as the max specs on the small Pent air system. I will offer that you can find a 400 watt heater that fits the small Pent Air module, it is the Hydor Theo. I use this on my 90 gallon as the tank is in the garage and even San Diego get a few cold days!
Bill
Oh, and I agree with the union advice. I have them between every element on my system - and you will appreciate having them.