Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
pH is not a stand-alone value.
There are buffers in the water that help to hold the pH at certain levels.
Carbonate and bicarbonate (KH test) are the biggest.
When the KH is high the pH tends to be high.
When the KH is low the pH is controlled by other factors such as organic matter (organic acids) CO2, other minerals such as phosphates or other things. The pH may be higher or lower, depending on what other factors are in the water.
Here is how I handle my tanks.
1) Fish are more concerned about the GH. (Calcium and Magnesium) so make this level right for the fish.
2) Make the KH pretty close to the GH. Within a degree or 2.
3) If the fish are black water fish add peat moss or other source of organic acids.
4) Let the pH do what it wants. When the GH and KH suit the fish, the pH is usually not too far off.
Sources of carbonate and bicarbonate:
Sodium bicarbonate (Baking soda). 1 teaspoon added to 30 gallons of water will raise the KH by 2 German degrees of hardness. When I added this much to a tank with pH off the chart low (6.0 or lower) the pH came up to 6.2.
Potassium bicarbonate. 1 teaspoon added to 25 gallons will raise the KH by 2 degrees. I have not tested the exact pH in these tanks, but it does come up, too.
Coral sand, Oyster shell grit, limestone sand or very fine gravel, limestone rocks.
These will add calcium, magnesium and carbonates, thus raising the GH and KH. The finer materials tend to react faster, and some rocks, even though they are limestone or related materials do not dissolve very fast, so results will vary.
Ways to raise the GH:
If you know the Ca and Mg are a reasonable ratio already, then add a balanced material like Seachem Equilibrium or Barr's GH booster.
If you know the Ca: Mg ratio is skewed, then add the needed material. Calcium chloride is one source of calcium, epsom salt is a source of magnesium.