lower pH

A smallish amount of Tannic Acid will leach from Bogwood but unless you have a small tank and a lot of wood the effect is negligible. If your water was already on the acid side
(pH below neutral - 7) which yours isn't, but at a lower pH level something even as slight as Bogwood could start off a 'pH crash', but you needn't be worrying about that.

You can achieve a very gradual pH reduction by using a very fine 'sock' - something like a ladies popsock, if such items are still available - filled with Aquarium Peat inside a filter, but once again starting from 8.5 it would take a long time even so. I prefer the Peat Granules like those produced by Sera for one as they are much less messy than the garden-type peat I used to use.

pH reduction can also be brought about with Acid such as Hydrochloric or Phosphoric but this stuff is dangerous and could prove fatal if misused, not only for the fish either - so I would caution against the use of that. It must always be remembered that acid, especially neat, must always be added to water. Under NO circumstances must water be added to acid - this would cause a really dangerous reaction which could see the person doing it very badly burned at the very least! All in all the acid route should not really even be considered!!!

There is stuff - pH Down which is supposed to drop the pH of the water to a given amount but I never found it to be of any use at all. All it ever achieved for me was to make the water extremely cloudy (which is supposed to clear after a while - the only way I got the water to clear was to empty it out and start again).

You could consider doing your water changes with filtered rainwater - which should really not be collected until the rain has been falling for approximately 30 minutes. This is something I use myself, but once again you really need to monitor your pH and don't let it drop too suddenly - slow and easy is the way to do any change in water composition.

HTH
John

The foot section of a discarded pair of tights or stockings (oops, showing my age there - again!!!) would be just as good, but as Derek suggests you might want to look into setting up a water treatment 'plant' such as an RO or DI - especially if you intend 'expanding' at any point in the future.

Some LFSs sell RO water which might be another option to consider.
My RO definitely drops the pH considerably...even though it does creep back up somewhat after being allowed to stand for 24 hours.

John