Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
GH is a test of calcium and magnesium.
KH is a test of carbonates and bicarbonates.
These materials can exist separately and at different levels in the water.
Usually, in nature, they are at least somewhat related for this reason: A lot of the earth comes from limestone and related materials (calcite, dolomite...) which are calcium carbonates and magnesium carbonates.
When rain water (especially rain with a little CO2 dissolved in it) runs over these materials they dissolve, and the calcium, magnesium and carbonates are added to the water. Usually in more or less equal amounts.
Biological processes can remove these minerals from the water. Fish and plants extract the calcium and magnesium from the water. Certain plants (about half the plants we use in the aquarium) can utilize the carbonates as a source of carbon. Nitrifying bacteria get their carbon from carbonates.
Ultimately the water that has picked up these minerals ends up in a lake (such as the rift lakes of Africa) and the water evaporates, leaving the minerals behind. These lakes are very high in these minerals, and we call this hard water. In aquarium terms we joke about 'liquid rock'.
This water can end up in the ocean, too. This is the source of many of the minerals in the sea water. Fresh water with low levels of minerals and salts flows into the ocean, and the water evaporates, leaving the minerals and salts in the water.
When people use the fresh water with minerals for household use the minerals can come out of the water, and cause white, crusty build up on the plumbing fixtures, and it is hard to work up a good lather in hard water. So municipal water companies often remove a certain amount of the minerals. In the home people can use a water softener to remove the calcium and magnesium. Usually this process (home water softener) adds sodium to the water.
Now, back to tests:
Check the date on your tests, 'new' is not when you bought it, but when it was manufactured. If it sits in a warehouse, or on a store shelf, it is aging.
GH is the shortest lived test. Most of them are good for 3-5 years, some may last longer. GH really does give up at about 3 years. The way it quits is to stop changing color, just as you are seeing.
Test this by...
using another test method (such as strips)
take a sample to a local store
read the water report from your water company.