Having just added a CO2 setup to my 120G, I now realize I should also have a CO2 level checker, but am told they are NOT all created equal! Recommenations for which one to get would be appreciated. Sponsors, feel free to jump in also...
IMHO, all drop checkers are the same, just pick one that you like the look of (and is in your price range).
the PH solution is bromo blue, same as what comes with most ph test kits. As for the reference solution se above (baking soda + water to make a dkh4 solution). This solution must be done accurately tho. Another option is to buy the solution locally, I know aquariums west in downtown has it (that's where I got mine).. else... check the sponsors on here
one more thing to consider is having more than one drop checker. sometimes you want to make sure your eniter tank is getting co2 evenly
Placement is indeed key, you'll want to place it somewhere that gets the average circulation current your tank will get and away from the reactor output. But i agree with jonney, 120 g will definately want more than one drop checker