I use two 2 liter bottles on a 65 gallon tank, but with low light, around 20-25 micromols of PAR. You probably have low medium light. I think a pair of 2 liter bottles will give you very noticeable results, and you will for sure need to begin dosing NPK to keep up with the increased demands of the faster growing plants.
The problem you may have is that it is hard to keep the CO2 production reasonably constant, even with replenishing one bottle per week. That may trigger some algae growth.