CO2 is CO2 Kelly,
Its not the mix that controls your levels but how it is introduced to the water column that does. Without a diffuser or reactor you need to keep a CO2 pocket in contact with the water at all times for it to dissolve and become a factor.
I was never a fan of the airstone method because it simply puts out bubbles that float free to the surface and dissipate.
Being such a small tank Im not sure how you could do that, maybe a pill bottle suction cupped to tank wall ? :roll: