Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
GH is a test that measures calcium and magnesium. These are essential to fish and plants. Plants use these at a level somewhat under the macros, but more than micros. In gardening they are referred to as secondary nutrients. Usually the tap water supplies these, and if the GH tests 3 German degrees of hardness or higher, then you can usually assume that the Ca and Mg are there in something like a reasonable ratio.
If you suspect problems, the you would do a calcium test (make sure it is for fresh water) then look up the formula to figure out what the magnesium is. If you need to supplement, then the plants use about 4 parts of Ca per 1 part of Mg. The ratio in the water does not have to be exactly that, but if you have to do something about it, then target that sort of ratio. If the GH is too low, then a GH booster such as Barr's GH booster or Seachem Equilibrium are good products. There are others, read the label. Some have sodium chloride, which you do not want.
Epsom salt will raise the GH, but it is only magnesium. If you know your tank needs just magnesium, and the calcium level is OK, then Epsom salt is great. But if you do not know what you need do not JUST add Epsom salt to make the GH test turn the right color. Your goal is to add the right minerals, not just make a test look good.
KH is usually just thought of as a buffer that stabilizes the pH.
The nitrifying bacteria can use it as a source of carbon, and roughly half the aquarium plants we grow can use the carbon from carbonates.
Usually the only reason you would alter the KH is if you want to alter the pH.
If the KH is low, then the pH is often controlled by something else, such as CO2 or peat moss. If it gets too low the pH can drop very low (below 6.0, IME)
If the KH is high, then the pH will almost always be high, and difficult to change.
How this relates to red plants:
Certain forms of Iron are easy for plants to use. Other forms are not well used. Chelation sort of locks up the iron in a way that keeps it available for the plants.
Chelated iron is available with several different chelators. Some of these last longer in lower pH water, some are better when the water is about neutral or a bit higher.
If you want the iron to be available for a long time (at least until the next dose) check the pH and use the right chelated iron.
Or alter the pH to use the other one.
Altering the pH opens another can of worms. Better to just use the chelated minerals (including iron) that is best for the pH you have.