Hi Paul.
The best thing to do before adding the bogwood to the water is to soak and spray it thoroughly.
I added a nice piece when I set my most recent tank up and before I put it in I used to boil it in water straight from the kettle for about 15 minutes, then I would spray it with a shower head or hose pipe. I repeated this every day for about 5 days. When I finally put the bogwood in the tank, it left no tannings.
So, I recommend you remove the bogwood now, follow the steps I did. Do bigger water changes in order to reduce the current tannings, then in a week add your bogwood and hopefully it should be fine.
Although I must say, the tint in the water i've seen on tanks gives it a lovely natural look.
Good luck, James.