Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Significance of GH and KH:
GH is a test of calcium and magnesium. This is the most important one for the fish. They thrive when the mineral levels are right.
There is no one 'right' number for GH. It depends on the fish.
Soft water fish (Amazon, Congo, several Asian rivers) thrive with a GH under 10 German degrees of hardness, and some really need it under 3 degrees.
Hard water fish (many live bearers, Rift Lake fish, some Rainbow fish) need the water well over 10 dGH.
KH is a test of carbonates and bicarbonates. Nitrifying bacteria get the carbon they need from either of these materials. KH is also a buffer for the pH.
If the KH is high, then the pH is usually high, and difficult to change.
If the KH is low, then the pH can be controlled by something else in the water, and may be high or low, but is often easy to change.
In nature the major source of GH and KH is the limestone formations, and related materials. These are calcium and magnesium carbonates.
Since the minerals contain both GH and KH materials, when they dissolve, the water will get all the minerals:
Other (there are always other things in the rocks and soils)
...in the ratios that these materials were in the rocks.
As a rough guide, the GH and KH should be fairly close to each other in the aquarium.
They are not always the same in nature, but it works for all the fish I have kept, to make up water this way:
1) Set the GH to suit the fish. A bit softer is OK.
2) Set the KH to match the GH. A bit softer is OK.
3) Test the pH.
a) Soft water, especially black water species: Filter the water through peat moss to add the organic acids the fish like. This also drops the pH.
b) all other fish: Add a bit of KH if the pH is too low. Add a bit of GH booster to keep the GH = KH.
When I first started making water this way I would run it overnight to be sure the minerals were all dissolved, and the parameters were stable.
I still need to run it overnight to filter it through peat moss for those few species, but mostly I just mix it up and use it. My water does not change parameters overnight. Half an hour to be sure all the minerals are well mixed is fine.
I use GH booster like Seachem Equilibrium or Barr's GH booster for GH. You can also make your own with Epsom salt and a calcium source like calcium choride.
I use potassium bicarbonate or sodium bicarbonate (baking soda) for KH.
Target for iron: 1 ppm.
So, if you are using a blended trace mineral supplement like CSM+B or Seachem Flourish Comprehensive you would test for iron, and assume all the other minerals are correct, if the iron is correct.
A target of 1 ppm is a general number. There are complications:
Some plants need more. Some do not like excess iron.
Iron and phosphorus fertilizers can interact, forming a precipitate that is not available to plants.