Hello, I'm new to the forums. Well, I just registered anyway. I'm familiar with the site, it's been a great source of ideas and information.
This has been working great so far, so I thought others might find it useful.
The problem with DIY CO2 is that you can't simply shut it off, so you have to adjust the yeast/sugar recipe and then count bubbles, which can be unpredictable. I've been experimenting with DIY internal CO2 reactors, and this is what I came up with. The idea behind this project was to make an internal reactor for DIY CO2 that can be turned on and off with a timer, and allows excess gas to escape to the surface, giving you some control over the amount of CO2 being dissolved into the water.
Unlike other similar DIY internal reactors, the powerhead can be turned off without CO2 accumulating in the impeller chamber, causing an air lock. The gas will escape out the side of the reactor instead of going up the outlet into the impeller chamber, so this one can be switched off and back on again without the need to re-prime the powerhead.
When it is turned off, it just functions as a "bell" type reactor that traps the CO2 in the bottle, with only a minimal amount of CO2 being dissolved into the water, and most of it escaping through the hole in the side and bubbling to the surface. Once it is switched on, the flow of water enables a much higher dissolving rate, and all of the CO2 coming in should get dissolved without any of it escaping to the surface. The photo shows it in action. It's hard to see the outlet hole in the side, but if you look closely you can see that the water/CO2 level stays about 1 cm above the outlet hole when it's switched on. It's kinda fun to watch it "breathe" as the level fluctuates slightly up and down (the CO2 entering and then dissolving).
This design should give you a fairly consistent amount of CO2 no matter how much gas your yeast/sugar mixture in the generator bottles is producing (if it's coming in faster than it's dissolving, the excess should just bubble out). This way, there's no risk of gassing your fish unless you're leaving the reactor on for too long. I have mine set to turn on 1 hour before lights on, and set to turn off at lights out.