CO2 is important. Whether you need to inject additional CO2 depends on the plants. So about the leaves turning yellow/brownish, are they new leaves or older leaves (any leaves from middle to bottom)?

Imo honestly co2 is not really important, what u do need is ferts, regular tap water does not contain much or anything at all. Fish waste alone also wont provide your plants with all the needed nutrients. Also lets not forget about lighting, sometimes lighting is not strong enough.