Well, Chlorophyll is the green pigment in plants which gives them their green colour. So, if there was a lack of chlorophyll (most likely due to a lack of CO2
in the tank), then that could definitely cause plants to go yellow in colour. So you're probably looking for something like this: Fluval Pressurised CO2 Kit
... if you want to get something to make your plants grow healthier.
You should be able to find one A LOT cheaper than that, however.