CO2 is usually not the issue, unless you are running a high-tech planted tank with very intense lighting and daily nutrient dosing. Diffused CO2 is often needed for this, but not for a low-tech or natural method planted tank.
The light is the primary factor, as light is what drives photosynthesis. The intensity and the spectrum are both critical; once stable, the duration can be used to tweak things. Data on the light may help us (someone, I am not up on LED), along with the plant species and numbers; a photo of the tank may provide this.
The Interpet Plant Food (if the link below) may be OK, though it does not give the nutrients and that always bothers me.
Easy, effective liquid aquarium plant food for fresh water and tropical aquariums, by Interpet, experts in aquarium care for over 65 years.
www.interpet.co.uk
The liquid CO2 I do not recommend; this contains a highly toxic disinfectant which can kill some plants even at recommended levels, and if overdosed could wipe out plants, fish and bacteria. Something like this has no place in a fish tank.
CO2 occurs naturally from respiration of fish, plants and some species of bacteria, but primarily from the breakdown of organics in the substrate. The fish excrement is usually more than sufficient in natural or low-tech systems.
CO2 diffused is also detrimental to fish, this is now being accepted more and more, so it is something I would avoid if fish are to be the focus. In an aquatic garden with plants but no fish, a different story, as you need a source of CO2.
