Check out what the directions say: "After first drop is added, solution will turn orange. If the water sample contains only l° dGH, the solution will turn from clear to its green endpoint after the first drop is added."
Your sample reached its endpoint (green color) after one drop. It didn't turn orange. So, the GH is 1 degree and not 0 degrees according to the test kit's directions. (Actually it could be between 0 and 1 degree as the test kit isn't very accurate.)
There are 17.9 ppm's in 1 degree or 1 ppm equals 0.056 degrees. This test kit isn't accurate enough to determine its exact ppm.
It has been so long since I used this kit that I couldn't remember what colors the starting point and endpoint are. I was thinking that orange is the endpoint and green was the starting point. This led me to believe that you haven't added enough reagent. It didn't make sense to me that you kept adding drops after you have reached the endpoint.
Orange is the starting point and green is the endpoint. I had to look it up. http://www.americanaquariumproducts....structions.pdf
Good luck with your aquarium. With your broad experience I don't believe that I will be of any further assistance.
(There are some typos after coping and pasting the directions from the pdf. http://www.americanaquariumproducts....structions.pdf
Directions lor Testing General Hardness (GH)
Read thoroughly before testing. Do not allow
Test Soluflons to get Into aquarium.
To remove childproof safety cop: With one
hand, push red tab left with thumb while
unscrewing cop with free hand.
1. Rinse a clean test tube with water to be tested.
2. Fillthe test tube with5 mlof aquariumwater
(to the line on the test fube).
3. Add General Hardness Test Solution, one drop
at a flme, holding dropper bottle upside down
In a completely vertical position to assure
unllormltyof drops. After first drop ~ added.
solution wUllurnorange. lithe water sample
contains only l'dGH, the soMIon wliturn from
cleor to lisgreen endpoint ofler the firstdrop ~
4. Cop the test tube and Invert several times after
eoch drop. Keep count 01the drops being
added. Do not hold finger over open end of
the tube. as this may affect the test results.
5. The test Iscompleted when the water Inthe test
tube, after having been shaken. turns from
orange to green. Ifyou have difficulty
discerning the color after the firstdrop of test
solution Isadded, remove the cop from the test
tube and while holding It over a v.t/Ie
bockgrcx.ncl bel<down 1Irough the tube.
6. The General Hardness value Isdetermined by
the number of drops 01the reagent that must
be added to turn the water In the test tube
green. Each drop Isequal to 1 'dGH or 17.9
ppm GH (see the chart).