If the Flourish Comprehensive Supplement is still a clear brown/orange colour, it should be OK. Refrigeration from the start is advisable, as you use so little the bottle can often last for months and after using this for over ten years now I find it does tend to deteriorate after a few months.
Since you mention Flourish Excel, I will just caution you that this is not a safe product to use for fish. It contains glutaraldehyde, a highly toxic disinfectant used in substances like embalming fluid, anti-freeze, and ship's ballasts to kill bacteria. It will kill some plants even at recommended dosage, and the fact that it may kill algae should warn us. If it should get overdosed, it can kill plants, fish and bacteria. It is used in hospitals to disinfect instruments, which should tell us something of its toxicity to living organisms.
There will be sufficient natural CO2 occurring in most aquaria from the decomposition of organics primarily in the substrate, plus the respiration of fish, plants and some bacteria.