I don't think the problem is CO2. When we measure CO2 by measuring the tank KH and pH that almost always gives too high a number, not too low. Here is an article on drop checkers:Drop Checkers/CO2 Indicators-Why and How. Here are some sources for them: Boyu Drop Checker Type 1 [5654] - £12.99 : Aqua Essentials,
Chame Co2 Drop Checker - Live Aquarium plant fish tank - (eBay item 250183686041 end time Nov-10-07 18:37:59 PST),
http://www.aquaticplantcentral.com/f...-checkers.html