Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
GH is more important to fish. There are some fish that cannot handle the higher calcium levels of harder water. There are many fish that cannot get enough calcium out of soft water.
Many fish come from black water rivers, and similar environments. These waters are high in organic acids. Bacteria seem not to thrive in these environments. The fish from these waters might have a somewhat less effective immune system because they depend on there being a reduced bacteria count in the water.
Here is how I would set up the tank(s) in question:
1) Research the livestock. Find out their optimum GH, and the acceptable range. Keep animals together only if they thrive under similar conditions. Make their tank water the right GH.
Lower GH by blending tap with distilled or RO.
Raise GH by adding THE RIGHT MATERIAL.
GH is a combined test for both Ca and Mg. The livestock need both minerals. If you are only adding one (Mg, for example, from Epsom salt) then the animals will not be getting the Ca they need, even if the GH test is turning the right color.
You are not altering the GH just to get the right color on the test. You are altering the GH so the water has the right mineral balance for the livestock.
Plants use Ca and Mg in a ratio of about 4 parts Ca to 1 part Mg. Fish are OK with this ratio, too. So, when you are setting your GH make sure you are using a product with the right balance of Ca:Mg.
If you think the tap water is not balanced this way, then get a fresh water calcium test, then research the formula so you can figure out the Mg.
2) I make the KH pretty close to the GH. I do this because in nature the main source of both carbonates and Ca and Mg is dissolving minerals such as limestone and its many related minerals. When this sort of rock dissolves the GH and KH of the water end up fairly close.
Blend tap water with RO or distilled to reduce the KH.
Add potassium bicarbonate or sodium bicarbonate (Baking soda) to raise KH.
Nitrifying bacteria use the carbon from carbonates. Many organic processes like decomposing waste also uses this. Monitor the KH in the tank, and if it starts to drop you may have to add more carbonates.
One way to do this is to keep a bag (nylon stocking) of limestone sand, coral sand, oyster shell grit or similar material in the filter. Note that these materials will also raise the GH.
When the GH and KH are in the right range for the fish then the pH and TDS are likely to be pretty close to the right range.
For black water species I add a nylon stocking of peat moss to the filter, and prep the water with peat moss in the barrel.
I am not looking for a specific pH, but rather the right mineral levels for the fish, and the right water chemistry with the black water or organic acids.