You're probably having trouble reading the GH test because it is changing color with the first drop, and the less drops needed for the color change, the fainter the color is.
TDS 41 ppm = 2.3 German degrees. Since this is the total dissolved solid reading, it not only reads Ca2+ and Mg2+ (GH), but everything else also. CO3, HCO3, K, Na, Cl, Fe, PO4, SO4 among others.
Since you don't have a problem reading the KH test, and the result is 1 dKH (17.848046 ppm), this leaves 23.152 ppm (1.3 dGH) of other dissolved solids (give or take).
In typical aquarium water, GH is basically only Ca2+ and Mg2+. If you have any K, Fe, NO3, NH3, PO4, SO4 (among others), this detracts from the 23.152 ppm of dissolved solids above. Leaving around 1 German degree (17.848ppm) or less of GH.
Last edited by Audionut; 07-05-2015 at 03:05 AM.
Reason: Clarity regarding ions