The CO2 you add with seltzer water quickly leaves the water at the water surface. That means you will have wild fluctuations in concentration of CO2 in the water, a good invitation to BBA to begin growing in the tank.
I use DIY CO2 on a 65 gallon low light tank, and find it is very effective. It wouldn't work for high light tanks but for low and possibly medium light tanks it does fine. I have a pair of 2 liter bottles, staggering the replenishing of the bottles, one per week.