Just as an update. I've got 2 Acid/Soda systems going. One for my 2.5G shrimp tank, and one for my 55G community planted. I'm using the OTHER system (the one that's just the off-white 2-liter caps with a pressure gauge, and tube running from one to the other), but the same formula. 200g Acid in 600ml water, and 200g Soda in 200ml water. I do not take any sort of special concern in getting it right. I actually just drew 2 lines on the bottles. 1 to indicate what 200g of soda looks like, and 1 to indicate where 200g of soda + 200ml of water ends up. Same for the soda bottle. So, just fill to the line with acid/soda, then top off to the 2nd line with water. I'm assuming this introduces a solid 5-10% margin of error in my formulas, but I didn't think it would matter much.
And, when running the 2.5G one at ~1BPS, for ~10 hours per day, for 8 days now, I've barely used any Acid at all. The gauge reads a tiny bit over 2psi, right now. Out of the green, but not enough to worry about, I think. But, it is seriously cool that I'm less than 10% from the top line. And, rough mathing it all out, maybe it will last for 2-3 months. Absolutely awesome, if you ask me.
On the 55G, pushing ~5BPS, for around ~9 hours a day, I'm having to refill once a week. The gauge has been around 1.5psi. Not great on refill time, but I just do it when doing water changes. No big deal. Until I can scrape up the money, I do believe this will cover my CO2 needs just fine. We've been pretty good at keeping the drop checker where we want it, as my wife is a stay at home mommy, and can pop in and check on both systems a few times a day.
I will say that without that luxury, it would be very hard to maintain a stable CO2 saturation, and pH level, using this equipment. It can fluctuate quite a bit during the say, we've found. Maybe a solenoid, so the horrible little valve wouldn't have to be messed with each time you turn it on and off, would help. Dunno.
Working for us, though.
-- 2.5 Gallon mini bowfront with a field of crypt wendtii, some java fern, java moss, 8 RCS, and several species of snails.
-- 55 Gallon long with Green Rotala, Crypt Wendtii, Red Ludwigia, Blyxa Japonica, Bacopa Carolinia, Banana Plant, Dwarf Sag, and an Amazon Sword. 2 Bolivian rams, 2 German Golden longfin rams, 2 German Blue Longfin Rams, 17 Rummies, 7 Neons, and a lonely Amano.