I think I found an inexpensive (as in really cheap) way to dissolve CO2 in a tank. It's so simple, it's probably been done before and the old timers in here will probably point out that they were doing it this way years ago. It's kind of a mix between a venturi and a good old ladder.
With time, I came to realize that CO2 is easy to dissolve in water. If the gas remains in contact with flowing water for long enough, it will dissolve.
All you need is the following:
1. A powerhead (with venturi input)
2. A standard T
3. Lots of airline (I'm testing it with just 6 feet, it's what I have laying around... more could be better)
4. A small piece of foam/sponge (like the sponge inserts in filters)
Here's a crappy plan, I have not included the checkvalves:
Some CO2 still makes it to the powerhead, but compared with the gas that is sent in the system, it's not that much. I think that if I use 10-15 feet of hose between the powerhead and the T connector, more gas could get dissolved, not certain yet...