Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
pH is not a 'stand alone' value.
Many of the things in the water might affect the pH. The 2 biggest are the carbonates, measured by the KH test, and organic matter including fallen leaves, fish food, fish waste, driftwood and so on. Adding CO2 to the water can also lower the pH.
If the KH is high, then the pH tends to be high, and difficult to change.
If the KH is low, then the pH is more easily changed, and is often controlled by something else in the tank, often the organic matter or CO2.
Water comes out of the tap with many minerals in it.
When the water is in a tank, and evaporates, only the water evaporates. The minerals stay in the tank. Then you top off the tank with more water, with its load of minerals. If you do small water changes, you are probably not resetting the tank to the original level of minerals, so there is a gradual increase.
The most prevalent mineral in the water comes from limestone. This is composed of calcium carbonate, magnesium carbonate and some other minor things. It dissolves in water fairly easily. The calcium and magnesium are measured by the GH (General Hardness) test. The carbonate and are measured by the KH test. Carbonates and bicarbonates interact in the water with CO2 and the pH changes.
The pH can be altered by accumulating minerals (especially carbonate and bicarbonate) in the water.
These can come from the tap water, or from decorative rocks, or from the substrate.
The pH usually goes up when the carbonates rise.
Most organic processes tend to lower the pH. But if the carbonate level is quite high this counteracts the acidity, so you may see a brief dip in the pH, but it will come back up.
To permanently alter the pH you need to reduce the carbonates to the level that they will not interfere with other things, then add other things that will lower the pH.
Most aquarium keepers will use a reverse osmosis filter to remove the carbonates (and pretty much everything else) from the water they have, then blend that water that has no carbonates with their high carbonate water until the KH is where they want it.
Most aquarium plants are pretty adaptable to a wide range of mineral levels and pH.
Most fish have a certain mineral range where they will thrive best. Decide which fish you want to keep, and set up your water to suit these fish. The plants will be fine.
Here is how I do it:
1) Set the GH to suit the fish.
2) Set the KH to match the GH within a degree or so.
3) If the fish come from black water rivers, add peat moss to the water prep barrel and to the aquarium filter.
Other than that, I just let the pH do whatever it wants to. It is not so important that I try to micro-manage the tanks to a particular pH. I use pH more as an indicator. If the pH is stable, then other things in the tank are stable. If the pH changes, then I look for what is causing the changes.