most co2 indicator will instruct you to use aquarium water to determine the co2 levels. from the colour changes ie blue (high ph) to green (mid ph) to yellow (very low ph), it will give you an indicator of co2 inside the tank.

Now, using aquarium water in the co2 indicator is ok, but less accurate (this is as i understand from much reading on the internet). to use 4dkh water, its a better way of telling the co2 being dissolved inside. reason being that 4dkh is a reference water (think universal where everyone is having same 4dkh water) while your tank water has got different ph compared to other people's tank. in fact, by adding the indicator solution + tank water, u r just saying your current tank's ph is so and so range, when the co2 is turned on, i should get to the yellow-green range to get the 30ppm target...
*please correct me if its inaccurate*

also, to make it clear, it is an indicator only. its not super accurate. it takes roughly 1-2 hours to get the colour changes. the tell tale sign is to look at your fishes. if they are breathing heavily + on the surface, its too much co2!! this can happen even though your indicator is still in the green level...

personally, my indicator has always been dark yellow. it does go to bright yellow at some stage but my fishes are still happy with it.

look for kiminha's explanation on why there should be 2x co2 indicator using different dkh solutions. u will get a more accurate reading of co2 in your tank.

btw, dun get confused between tank's ph and the co2 indicator's ph. adjusting the tank's ph does not give you co2! injecting co2 and change your tank's ph using other methods are 2 different things!

cheers.

oh, over 2500 posts, who wanna do v12?

well, most of us see from LFS and then read up then seek advise (from LFS n counter check online) then only purchase. yes, they have a business to run but again there are responsible business people too.

best you get yourself equipped with info that you need.