Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
KH is carbonates. This is the buffer that stabilizes the pH.
GH is general hardness. This is a measure of calcium and magnesium. These are essential minerals for plants and fish.
pH is not a stand alone value. The minerals and other things in the water dictate what the pH will be. By controlling things like carbonates, CO2, and organic matter you are controlling the pH. The fish are not so picky about the pH, they can handle a wide range and do not mind it fluctuating.
They want a certain level of minerals, and these mineral levels ought to stay stable.
Both GH and KH tests read in the same units. But they are not testing the same thing, nor do they overlap.
One test is KH.
The other test is GH.
1 drop of reagent represents 1 German degree of hardness in either test.
If you want to convert that to ppm then 1 degree = 17.9 ppm.
1 drop of reagent to change the color in the KH test means 17.9 ppm carbonates. You can write that as 1dKH or 17.9 ppm.
To raise the KH add carbonates or bicarbonates such as baking soda or potassium bicarbonate.
2 drops of reagent to change the color in the GH test means 35.8 ppm of calcium and magnesium. You can write that as 2dGH or 35.8 ppm.
Raise the GH with Seachem equilibrium.
So, your test so far has shown about 2 dGH, so I would add another 1/8tsp. for most average fish. I find it easier to mix in a small jar with a tight lid and some hot water. Then pour it into the tank.
Soft water fish like the GH to be not much higher than 3 degrees. Lower than this may also show deficiencies in the plants. Fish from very soft water include Cardinal Tetras, Chocolate Gouramis and many other specialty fish.
Most average community fish are fine with a GH from 3-9 degrees. Most of the fish you see in pet stores are this sort. Many Tetras, Barbs, Rasboras, Platies, Endlers and so on.
Hard water fish require a GH over 9, and as high as 20 is just fine. In pet stores Guppies, Mollies, Swordtails, Platies and 'Mixed African Cichlids' prefer this sort of water.
There are many fish not commonly found in most pet stores that are worth searching out. Do the research and fine tune the water to suit them.
Plants are (mostly) OK with any of these. There are a few specialty plants that really do require low GH, but these are not so common.
Once you have figured out what fish you want to keep, set the GH to suit them.
Then make the KH about equal to the GH.
When you have set the GH and KH like this the pH will usually be in the right range for most fish.
If you want a fish that comes from black water streams, then add peat moss to the filter.