Welcome to TPT!
First of, do not waste your money on Fluval 88. It has multiple problems and you will end up spending a lot of money on the refill cartriges. And I speak from my personal, hands-on experience.
I do not know your experience with planted tanks but there are many many plants that grow great with no added co2. If you want to go a step up, then suppliments like Excel will certainly help.
Injected co2 does not optimize plant growth all by itself, it is just one component of an inter-depended eco system that also includes light, fertilization, your water parameters, circulation and so on.
If you could describe your "ideal" tank and what you have now, then we will have a starting point for a discussion.