I expect the API test kit you have is the same as the one we have in NA. So to answer your question on the chart, the first column is obviously the number of drops it took to change colour, the second is the dGH or dKH this represents (you will see that every drop equates to 1 degree) and the third column is the equivalent ppm (parts per million). Just so you know, to convert you use 17.9, and multiplying degrees by 17.9 will give you the equivalent ppm, and dividing ppm by 17.9 gives you degrees. These two units of measurement are the most common in the hobby, some sources use degrees, some ppm, but it is easy to convert and you can use whichever you prefer. I like degrees as the numbers are smaller and for me easier to remember. If my tank tests at 4 dGH, I have a better grasp of what this means than if I saw 71 ppm.
I think there may be a misunderstanding of "Alkalinity" here, so I will explain. Hardness for our purposes is concerned with three things, the dissolved mineral content (the GH), the carbonate or bicarbonate hardness (KH, now often seen as Alkalinity) and the TDS (total dissolved solids). We can forget the latter for our purposes in this thread or it will really get confusing. The GH kit is testing GH or the mineral content, and the KH kit is testing carbonate hardness or Alkalinity.
There is also pH, and this may be where things are getting confused. The pH can be said to be acidic (pH below 7.0), neutral (7.0) or basic (above 7.0) and this basic used to be termed alkaline. This is not the same as Alkalinity or KH. Keep them separate.
Generally, soft water will have a lower KH and thus the pH tends to lower naturally due to organics. So over time, an acidic pH will normally be present with a low GH/KH. The harder the GH the more likely the pH will be more basic than acidic, though not always. The KH or Alkalinity acts to buffer the pH, preventing it from fluctuating or changing, so the higher the KH the less chance the pH will shift from where it is to begin with in the source water. However, these are generalities, and there can be different situations. Again I will leave this as it will only bog us down.
So to your readings. A GH and KH of 3 degrees, equating to roughly 50 ppm, is soft, almost very soft. I don't know what the pH of your water is, but in an aquarium with fish it will tend to lower as the breakdown of organics creates carbonic acid. This is normal, nothing to worry about. But if you see the pH in the aquarium lower than the tap water, this is almost certainly the reason.
The change of colour in the API test can be a bit difficult. I had a kit that was very hard to discern; I bought a new one and the colours are quite vibrant by comparison. Hold the open tube over a piece of white paper and look down through the water in the tube; this is usually the best way to see the colour. And the drop which changes it first even if light, is the result. If you are on municipal water and the authority has a website, you may find their numbers for general (total) hardness, Alkalinity, and pH. This would confirm your test results, though pH can change so don't be worried. The GH and KH should be the same or very close out of the tap. In the aquarium these can change, though not by much unless you are targeting them with calcareous substances or something.
Byron.
