02-14-2010, 03:48 PM
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Wood may slightly lower the pH but not significantly. According to what I have read, a drop of .2 [e.g. from 7.8 down to 7.6] is the most, and this only if there is nothing else raising it (such as calcareous rock/gravel). So this is not a reliable method.
A caution on the wood, if it is soft to the touch and when you wipe your finger across it a thick brown smudge/stain occurs, it is probably rotting and should be removed from the aquarium. It may or may not be detrimental to the fish, but there is always the risk that something deep in the wood may begin to leech out as it rots.
The two reliable methods to lower pH are peat filtration and RO water. Peat filtration involves placing a quantity of peat (you buy this in the fish store or elsewhere, other members will have more info) in the filter or in the tank and it slowly acidifies the water, turning it a brownish/yellowish tint at the same time. The amount required and its active period depend upon the hardness; it does have to be replaced from time to time.
RO water involves the use of a RO filter that removes minerals and such from the tap water, resulting in relatively "pure" water which is then mixed with a bit of tap water to restore some mineral. Here again, those who have used this method can supply more details if you ask.
Rainwater can also be mixed with tap water, same idea as above; provided you live in an area with little or no industry, and collect the rain not from a roof but in a barell so it doesn't pick up other contaminants.
Chemicals to lower pH will likely not work due to the degree of carbonate hardness in your water. Using them will lower the pH then the carbonate buffer in the water will raise it back, and so forth; this fluctuating pH is more stressful on the fish than leaving the pH as is, and may kill them.
It would help to know the KH (measure of carbonate hardness) in your tap water; this will tell us how much softening will likely be required.