You can go to your aquarium supply where you can look for some KH booster (must of the time it is CaCO3) go with the instruction.
You can find baking soda anywhere
To raise both GH and KH simultaneously, add calcium carbonate (CaCO3). 1/2 teaspoon per 100 liters of water will increase both the KH and GH by about 1-2 dH. Alternatively, add some sea shells, coral, limestone, marble chips, etc. to your filter.
To raise the KH without raising the GH, add sodium bicarbonate (NaHCO3), commonly known as baking soda. 1/2 teaspoon per 100 Liters raises the KH by about 1 dH. Sodium bicarbonate drives the pH towards an equilibrium value of 8.2.
This is for ianiwane, I agree that 6.4 is still OK for plant, but some nutrients absorption in plants start to decrease around 6.5. This is the case for (phosphorus, calcium, magnesium, molybden)
The only nutrient absorption rate that is actually help by lower PH than 6.5 is Iron passing from around 88% absorption rate to 100% absorption rate at 6.
You have no advantages to go lower than that only in very specific situation
At ph 6, phosphorus absorption is only around 25% to go up to around 98 % at ph 6.5
Last edited by Glouglou; 01-19-2007 at 03:02 AM.