I agree with wasser... I am trying to save money and I like to tinker around with projects like this... However the venturi valve is a good idea.
At this point my reactor works flawlessly and I have under $30 into it....
120g, Flourite & Sand. DIY Stand and Canopy. CF lighting 3wpg, Pressurized C02, DIY reactor. 2-XP3's, 500watt heater.