It will mainly depend on what your starting Ph is.
High Ph above 7.5 will probably need RO water to get down to below 7.0.
If all you need to do is take it down a couple points like from 7.2-7.0 Tetra will work fine or even just boil 15 large oak leaves for 15 minutes then put the liquid into bottles/jugs for use later.(leaves after boiling can still have some tannins and can be placed directly into the tank)
Peat can also be used but again it will take a good boiling in order to work and only takes the Ph down a little.
The coloration of your tank water will get murky or at the very least lightly tanned (with leaves or peat). If this bothers you don't use them.
Also you should strain the peat or leaves from the water if you don't want anything floating around.
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Last edited by Imi Statue; 01-11-2013 at 04:44 AM.
Reason: added info.