There are alot of ways to do that....and thoughts. The main ideal is to keep the Co2 bubble in contact with the water for as long as possible, giving you a asorbtion rating. The other things I have found comes into play, are the size of the tank and what method you are using to create Co2 gas, yeast or Co2 injection. If it yeast I would just either a good ladder or a power head. The reason behind my thought on this is the tank size should not be more than 30 gal tank for a yeast system. These tanks are small enough sze that a large in tank reactor take up usable space. Redsea has one that is the smallest I've seen, and I'm told it works effectivly if you keep it clean out. In larger tanks to generate enough Co2 gas say for a 55gal with yeast to be effective, it would take at least 6 two liter bottles linked together with no leaks. Also, after building it you have to wash and refill it every 2 weeks to a month, thats alot of work. Don't forget about the mess if it get knocked over. I use a yeast system on a small 29 gal tank, but I bought the systems becasue the hang on the tank and no chance of spilling.( Ihave cats and kids) I use two linked together using brass fitting and Co2 air line, into a the tank and use a ladder to diffuse it. The little indicator shows enough Co2 present and this tank is moderatly planted and grows will. Now if you using a Co2 injection system, I use a reactor in one tank and a inline reactor I built, if truth be told I like to say the inline was best but it is not, it works and works will but I prefer the in the tank reactor, it seems to use less gas. I use one and I can not see how any Co2 is lost from aborbtion. I also use a Ph controlers to hold my Ph at 6.5. In a 55custom tank. it very tall tank and very deep so hiding the reactor is not that hard. The other is on a 75 gal system that has an inline reactor. I used that reactor because I have kept everything outside the tank so all I have is a suction line and the return lineon the tank. So the diffussing systen you choose also need to fit the need for the tank.
It kind of depends on the size of your tank and your filter/water circulation. I used the Red sea reactor for my 20g tank:
It makes a cyclone out of the CO2 and dissolves it in the water which is then shot out at the bottom. Other gasses like O2 and Nitrogen don't dissolve and escape out the top every now and then. I wasn't sure if it was strong enough when I move to a 75g tank but, it had no problems bringing my pH down from 7.0 to 6.3. I guesstimate I have about 25ppm of CO2 in my tank. It does need to be cleaned out about once a month.
Others use glass diffusers and many of them bubble them into their filter's intake so that they get even more dissolved and circulate around the tank better. Others use bubble ladders. I would only recommend that one for tanks smaller than 29g. Some plumb it into their filter intake. This means one less thing to have in your tank and uses the filter to circulate it.
I have the first diffuser linked in djrichie's post above - the CO2 reactor 200 with rio pump. That thing works great and I highly recommend it. I've had mine for years. When the round sponge in the bottom wears out (this takes a looooonnng time) I use one of those plastic netting pot scrubber things and fit it into the bottom. This allows the CO2 bubble to bounce around a long while before dissipating. Great product.
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