My guess is that it will take about 3+ hours for that drop checker to read correctly. The size of the opening into the air chamber above the fluid is small and that will also increase the reaction time. The photo looks like it is out of the water - was that just to photograph it? It does have to be completely submersed in order to keep the solution and the drop checker at the same temperature as the tank water.

If you can find a funnel with a larger opening, or a smaller jar to use with the present one, that might help.