I built the venturi diffuser last night. I am not sure its doing what it should. Should there be a space at the top filled with CO2? If not, then I don't see what the tube from the side to the input of the powerhead is for??

What I have is maybe 1/8" of CO2 at the top of the big tube (which was from a big-al's vaccum thingy).

What happens is bubles come in near the bottom, get swirled around A LOT until the dissappear. Nothing much seems to happen re that extra tube???

Sounds like it's doing the right thing.

The tube on the side is to count the bubbles and to provide input for the CO2.

The hole on the opposite side from the powerhead is the burp hole, this releases excess gas if needed at night/adding too much for the reactor capacity.

The venturi loop, the tube connected to the powerhead, the the gas bubble will build up over the course of a day and then that part will kick in once it reaches further down and collect more gas. Then the gas will be sucked into the loop and produce a fine mist that's explled out the bottom.

Sounds like you are not adding enough CO2 gas to the unit, you should see mist at about 1/2 way through the light cycle.

Regards,
Tom Barr

I'm thinking about converting my powerhead/sponge filter/CO2 mist generator to this venturi diffuser design, with a sponge filter on the water inlet. This would be for when I switch tanks from a 29 gallon to a 45 gallon. But, my powerhead output will dig up the substrate now if I direct it towards the bottom, so will the output of this reactor do the same, for the same water flow? Will this diffuser work as well if I add an elbow at the bottom to direct the output more parallel to the substrate?
The reasons I am looking to do this are:
I'm getting a bit tired of the tankful of champaign look.
I'm not at all sure my Maxijet 600 will successfully spread CO2 mist in the new tank.
I think I can get a more uniform CO2 ppm with a diffuser, with a higher ppm, with less CO2 used.
I like to tinker!