I made this one for my wife, after purchasing an aquariumplants.com C02 reactor. I felt there where some changes I would make to their design, so instead of spending the $138 plus shipping and handeling for the version with a probe holder i made my own. Changes include moving the wire entry to the back of the unit to leave the front open to add my delivery ports. I also decided to put the ports on the front unlike theirs so i would not loose my purge valve( nice to have when trying to prime your filter) I also use two delivery ports instead of one like on their units, because i use a pinpoint ph controller which gives me the ability to not only add C02 but also add Oxygen if my ph drops to much(nice feature VS the Milwaukee controllers) all said and done i spent about $80 and thats only because i had to by a couple NPT taps(to thread my entry ports) I didnt already own. I also added a gate valve port so that i can use a wet vac if i am still having any issues getting the filter primed(comes in handy from time to time). The original version by Aquariumplants.com is still a great version and the one i own is still running strong. Just figured since i like to tinker and save money LOL id build my own version.