Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Discus and other soft water fish are really looking for the right mineral levels. The pH is secondary.
If you can test the GH, KH and TDS of the water then we can make more suggestions.
GH is General Hardness. It is a measure of calcium and magnesium. In the natural water of Discus and many other fish the GH is very low, and keeping them in an aquarium the GH should be not more than about 3 German degrees of hardness.
KH is a measure of carbonate hardness. If the KH is high then the pH tends to be high, and difficult to change. If the KH is low then the pH will be controlled by something else in the water, and can usually be easily changed. When keeping soft water fish the KH also should be quite low.
TDS is total dissolved solids. It is measured with a meter that usually reports the amount of all sorts of things in the water as if all the things were salt. More salt = the water conducts electicity easier. More stuff (minerals, salts) in the water and it will conduct electricity better, so the TDS meter reads higher.
The natural water that Discus and many other fish come from is almost pure rain water. So much rain falls that any minerals dissolved in the water are greatly diluted.
The rain falls through the trees and runs through the fallen leaves and decaying matter in the rain forest. It picks up organic acids from the plants, so the pH is usually quite low.
Here is how I make this sort of water:
Reverse Osmosis systems take out about 99% of everything in the water.
Then I add just a little bit of tap water back to the RO water so there is the very small amount of minerals these fish need.
The GH and KH of the water ends up at about 2 degrees.
Then I keep a stocking full of peat moss in the garbage can where I mix the water, and pump the water through the peat moss. A knee-hi stocking of peat moss is fine for 20-40 gallons of water and can be used several times.
The end result is
GH- 1-2 German degrees of hardness.
KH- 1-2 German degrees of hardness. It could drop even lower and that is OK. There are substrates out there that will remove the KH from the water.
pH between 6- 6.5, the actual number is not critical. If all the KH is removed the pH drops to the high 5s. Hobby test kits do not go this low.
I did not have a TDS meter when I was making this sort of water, but I think the TDS would have been about 100.