I have a 1 GA nano tank that I keep at my desk at work. I have an Azoo palm filter and a CO2 natural plant system with a little diffuser plate on it. The C02 tank is just a small canister about the size of a 20 ounce bottle of pop and the diffuser plate is a small, flat plate that I hide behind a wall of vertical stem plants. The rate of C02 bubbles is very low because of the small size of the C02 tank, I would say, one per 5 seconds. I have kept 3 amano shrimp and 3 neon tetras in it for about 6 months with no problems. The slow rate of C02 bubbles and the low efficiency of diffusion into the water makes it seem unlikely to me that I will have a crash, although I've never tested the range of PH fluxuation. The use of an additive like Flourish Excel didn't occur to me, because I cycle about 1 coffee cup worth of water every day, which would mean most of it would be wasted.
Everyone at work loves the tank though and comes over to talk to me about fish all the time. Most of them want to know what is wrong with their Betta, heh.