I really wouldn't think that a 120g stocked how you've had it would have terribly ph fluctuations unless you never cleaned your filters. If your wood has a lot of tannins, then it's possible.
Using a ph conditioner like that isn't something you use to get it stabilized, then stop. If you start using it, you use it forever, to keep your ph at 7.0 all the time. You slowly bring the ph up to 7.0, then you always bring your water change water to 7.0 before you put it in the tank. IMO this isn't the answer... but it could be.
I think bi-weekly 40% water changes SHOULD be ok, but you still may want to take some of the wood out of the tank. You could try boiling it for a while to get some of the tannins out...
Using a ph conditioner like that isn't something you use to get it stabilized, then stop. If you start using it, you use it forever, to keep your ph at 7.0 all the time. You slowly bring the ph up to 7.0, then you always bring your water change water to 7.0 before you put it in the tank. IMO this isn't the answer... but it could be.
I think bi-weekly 40% water changes SHOULD be ok, but you still may want to take some of the wood out of the tank. You could try boiling it for a while to get some of the tannins out...
