readings are likely erroneous if you aren't supplementing. CO2
concentrations in unsupplemented water columns equilibrate to somewhere around 2-3 ppm CO2
naturally. Measurements indicating higher levels are usually incorrect. Either your pH or your KH
test kits are giving you false readings if your water was measured after more than 24 hours of sitting in your tank. Measurements directly from tap water may display high levels of CO2
(lower initial pH readings) but, after 24 hours of atmospheric exposure, CO2
levels will have dropped significantly.
Seachem's Flourish Excel is a decent substitute for CO2
supplementation in low light tanks. It provides photosynthesis precursors to the plants but is not nearly as effective as actual CO2