Never ever follow he instructions coming with a drop checker. Instead:
Make or otherwise obtain some standard 4 dKH water.
Add enough of that water to the DC to fill the "bulb" about half full.
Add to that 2-3 drops of pH reagent - the kind that is blue at high pH and yellow at low pH.
Hang it in the aquarium not close to where the CO2 enters the tank.
Wait about 2-3 hours, and look for the color to be green to yellow green.
