I used this basic setup, minus the bio balls http://www.csd.net/~cgadd/aqua/co2.gif
Parts:
$3.99 blue glass vase and
$3.50 for 6"x36"x2" sheet of foam at crafts store (plenty left over)
$9.99 for 1/4" glass cutting bit
$4.95 for ceramic/glass grinding dremel head
$0 tubing,
$0 Power head, and
$0 for air-stone I had laying around.

I don't have my CO2 bottle yet and waiting on my solenoid valve, but I think I designed it too well. I blew into the tube and the air absolutely will not escape my reactor. As you can see, an air bubble is created at the top and doesn't escape. Though I found out a bonus when the air gets low enough, the incomming watter churns up bubbles even more. The problem is this...if I had a CO2 canister with bubble counter setup, it would continuly fill with air until there was nothing but the sponge at the bottom.

Just for an experiment I put a hose in the air inlet mister built into my powerhead, letting large amounts of misted air in. Still with the entire reactor full of air, nothing would come out the bottom. It was weird! Some how when the air got to the foam, it let the water by with out pushing out the air.

Pros:
-excelent exposure time of CO2 to water since the CO2 has no place to go but get spun up by the water.
-The air inlet on my power head would make an excellent place to hook up my CO2 suply to. The effect makes a much finer mist of bubbles then my air-stone
-With the CO2 being exposed for so long, I suspect my CO2 bottle will last a long time.

Cons:
-CO2 build up has no place to go if it doesn't get absorbed into the water.
-can't think of anything else.

Possible Solutions:
1) illiminate the air-stone, using the mister as the CO2 input and cut the tube connecting the airstone much shorter that so when the CO2 level gets to the end it will escape out the top.
2) Cut down the foam from 1" to 2" thick to possibly let some of the CO2 get through
3) Maybe the sponge isn't porus enough and needs to be so some of the air can escape with the water.
4) Get a more powerful pump so it forces the gas through the bottom.
5) If I tip the reactor sideways, a small strip of air forms at the top until it has enough presure to push by the foam and out.

Any suggestions?

2), 3) With foam that dense, CO2 bubbles won't be able to escape. Even if you make it thinner, I doubt you can get CO2 bubbles to come out
4) That could be a possible solution, but again, I think the foam is too dense
5) You'd get periodic "burps" of CO2.

I don't know if more flow through the pump will make the co2 dissolve faster. Possibly it will create more turbulence thereby increasing hte rate of dissolution.

have a read of https://www.plantedtank.net/forums/di...eactor+venturi this thread for an idea!

By the way, I just made a very similar reactor that doesn't have the sponge....and I still get the air buildup at the top. so I don't htink it's the sponge

These work VERY well, I decided to buy one for $30 already made because it was smaller, but that should do the same thing. 100% diffusion!

thank you for the Ideas.

I took the foam out from the bottom, took the difuser and tube out. This left only the power head blowing in, a little hole at the top for accumulated air to leave and a gaping hole at the bottom. I hooked up the veturi tube that is normally used to let air into the power head. I simply put my thumb on it and let only a little bit of air (barely any) and I got a fine mist inside the reactor. Any accumulated bubbles went out the top.

I will put a T fitting at the misting input of the powerhead. One side will be Co2, the other will come from the top of the reactor to recirculate any Co2 that accumulates. I am going to leave the bottom open and keep the sponge pre-filter on the intake of the powerhead as it slows down the output just enough to keep the bubbles from swirling out the bottom. Some still escapes, but not very much at all. Plenty of time for the Co2 to absorb into the water. I anticipate this to be a very efficient reactor.

Thank you, I just needed a little bit of info to complete this bad boy.
AND, I just got my Solenoid valves today!

OK, It's all up and running now! Here are pics of the improved setup

I made a feedback loop to mist the undisolved Co2 at the top



Here is a pic with my homemade bubble counter. It's crude I know, but It's made out of extra stuff I had lying around. It also is "burping" in this shot.


I put a piece of plastic used for needle work at the bottom. Got a sheet of it at the craft store for $0.39!


The reactor will "burp" every once in awhile when the Co2 at the top gets sucked back into the venturi tube at the top.


I think it has turned out very well. Only the very very fine Co2 bubbles get out the bottom. You can see fine bubbles floating around everywhere inside the tank. they eventually stick to the surface of everything including plants and even the surface of the water. They last for alot longer then you would think they could. The reactor is filled with tiny bubbles and sometimes some large ones when it "burbs" but it only gets churned up again and again untill it either is disolved or has gotten small enough to slip out the bottom. Just looking at it I would guess maybe 10% of the fine bubbles even get out the bottom. There is a slight backpressure created at the plastic mesh from the water trying to escape. This seems to repel the lighter bubbles and they simply stay in the reactor. It's my bubble force field The reactor is so effiecient that I only have to set my bubble counter to one bubble every 2 seconds or so.

Here is an example of how small the bubbles outside are with respect to the ones inside. If you look, the blue tube is 1/8th inch inside diameter tubing. That makes them less then 1mm, probably less then .5mm: