Forgive me for being lazy; however, I'm looking for a low tech way to measure CO2 levels and would like to know the safe range insofar as fish are concerned. Currently, I'm running a little DIY CO2 setup with a betta being the only resident. The plants are beautiful and the betta isn't bothered as he breathes from the surface anyway. Any info would be appreciated. I'd like to try the DIY CO2 in a couple of other tanks to see the reaction of the plants but, I don't want to harm the fish.