I agree O2 is important as I stated. I have had my Co2 levels drop from to much surface turbulence, I figured out what my tank needed by adjusting slowly until it stabilized.
I wouldn't go as far to say drop checkers are not accurate. Any measurement or test is only as accurate as the person using it. Obviously you wouldn't put a DC above the diffuser or in a dead zone with no current. As far as method of diffusion whether its inline or directly diffused in the tank, the Co2 level will diffuse with the water in the tank, it is a gas therefore it expands weather or not there is great flow or not;although flow is important for aiding diffusion and maintaining an even flow if Co2. (This is why you can spray one puff of air freshener in a room with no air flow or very little and it will expand into the whole house, however this process will be expedited by a fan).
Many people use their fish as a method of finding C02 limits. I don't like to use my fish, as stressing them out is unnecessary, and its not accurate. Fish can become acclimated to Co2 levels and other water parameters, therefore I cant trust that I have to much or to little or the right amount. This is why many of us still use the drop checker, bps, C02 Charts, and our inhabitants to gather as much information as possible in an effort to be as accurate as possible.
The KH in the tank water may vary but the KH of the drop checker solution is always 4dkh, so the solution is not affected by the tank water only the C02 off gassing in the drop checker. BTW I stated lime green as 30ppm is recommended, your light and plant load must be factored into the tanks demand, especially as your plants grow, so a yellow color may be what you are shooting for; as is the case for Aquaticfan.
GL with the tank ! Here are a couple of links to help:
Adding O2 back into the water is just as impurtant as Co2. Benificial both ways. As I posted ive actually used HOB filters to help disperse the Co2 coming from my diffuser and I get my drop checkers to turn bright yellow doing so.
Also as a note.. Drop checkers are FAR from accurate. It can read lime green or even yellow but that doesnt mean you have 30ppm of Co2. Drop checkers color and how fast they react all depend on placement, current, flow of co2, method of diffusion or such as a reactor. Some ways dissolve Co2 100% others dont. KH of the water and more. While I use drop checkers Really what i look at is my fish and plants. I push my Co2 til my fish say.. NO MORE and are stressed. I back it off till they go back to normal and then I back it down a hair more for safety. This would be the max Co2 level you really can attain without wiping out your fish. Personally I dont worry if its 30ppm or not. If my fish are happy and the plants show the right look and growth then its good enough. Example of this would be my tanks where the fish are happy plants pearl and grow, low to no algae and drop checkers read bright yellow.