Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
It would be an interesting test. The balloon would have to be flexible enough to freely expand under the low pressure of the yeast production. Then strong enough to force the CO2 into the tank when you switch it over in the morning.
In general, the yeast production method does not build up excess CO2 in the tank overnight to the point of poisoning the fish, but just a little excess to give the plants a boost in the morning.
An older style of diffusion included an upside-down bowl shaped thing above the CO2 outlet. CO2 that does not diffuse into the water bubbles up and is trapped by the bowl. Water movement under the bowl slowly picks up this CO2.
Put the yeast on top of the light, where it is warm. During the day, the little bit of added heat might make the yeast a bit more productive. When the light goes out it cools off.
I have no idea if the difference in temperature is enough to affect production. (I suppose this depends on the fixture) I am also aware that the timing of increasing and reducing production from the light is not the same as what it should be- CO2 ought to come on ahead of the light, and can be turned off before lights out.
I think the best way to help out the yeast system is to have a really good way to diffuse the CO2 into the water.