Planted Tank Guru
Join Date: Jun 2004
Location: Baton Rouge, LA
One 2L bottle, even with an aggressive yeast recipe that burns out in a week, is woefully inadequate for an 80G tank.
Even two 2L bottles will not be enough. Especially if any of the CO2 bubbles from the airstone reach the surface undissolved.
For that size tank, I'd estimate two or three one gallon bottles. And make sure the bubbles from the airstone get sucked into a filter intake to ensure better diffusion.
But no matter what DIY setup you have, you still need some way to determine what your CO2 levels are, to verify they're adequate. Preferably a drop checker and 4°KH solution, which is cheap enough that you can probably buy it now. I think someone sells them together in the Swap 'n Shop here. And it will still be useful when you get pressurized CO2.
Otherwise, if you have PH and KH test kits, you could use a CO2 calculator to get a rough estimate of CO2 levels from that.