Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
KH of anywhere above about 2 German degrees of hardness is just fine.
Baking soda is the most common material used to raise the KH.
1 teaspoon per 30 gallons will raise the KH by 2 German degrees of hardness, and will raise the pH a bit. Baking soda can be added directly to the tank, but I put it in right in front of the filter outlet so it dissolves best. It is OK if a bit lands on the floor of the tank, it will dissolve. You could also stir it into a glass of water and add that to the tank. It will be a little bit cloudy for a short time. Very short time, IME.
There is not a material that will raise KH without raising pH. Do not worry about pH.
GH is a measure of calcium and magnesium. If the GH of the tap water is under about 3 German degrees of hardness the water may be lacking one or the other, or both minerals. Can you get a water quality report from the water company? (Or are you on a well?)
You can add GH booster, a blend of Ca and Mg. Equilibrium is one brand, and it also has potassium in it, and some trace minerals. All that is fine. This one will not affect the KH. This one is hard to dissolve. I shake it in a sealed jar of water, pour into the tank the milky water, then add more to keep dissolving more of it.
Another way to raise both GH and KH is to add coral sand, oyster shell grit or limestone sand to the filter. I use nylon stockings for this. These materials can raise all 3: GH, KH and pH.
A good recipe:
Make the GH what you want for the fish.
Make the KH about the same, plus of minus a degree or so.
Let the pH do what it wants.
If you are keeping black water fish, then filter the water through peat moss, or add Indian Almond Leaves or other materials that will add the organic acids that those fish like.
Most aquarium plants are not very picky about the mineral levels as long as they are not too low. GH and KH of 2-3 degrees is about as low as I would want for most average plants. There are some specialty plants that demand soft, acidic water. Carnivorous plants are one group, and there are others.