Oh I have a funky diffusion method. I have an Aquaclear10 powerhead with the CO2 feeding into the intake. On the outlet I have some funky irrigation part, which is basically an L-bend that directs the bubbles downward. I position the L-bend slightly at an angle so that the bubbles rise into the outlet of the canister filter. So basically the tiny mist I produce (finer than the mist out of a ceramic diffuser mind you) is shot down then floats back up only to be pushed down and across the whole of the tank. It gets the job done lol.
I think the overdose happened when I topped off the tank too high so that there was 0 surface agitation.
My mix is 1/4tsp of red star yeast, 2 cups of sugar, 2l of water and 5ml of pH up.
Tank volume of 20g.