Varnishing or sealing wood is a bad idea. Some fish like to eat wood or the algae growing on it, and they will scrape off the varnish and posion themselves. A plec is definitely the sort of fish that will do this.
The old trick for removing the tannin from wood is put it in the cistern of your loo. Each time you flush, it'll get a nice rinse, and between flushes will have the chance to leach out any chemicals. This is a great approach for cleaning sea shells and other stuff collected from the wild, too. Anyway, do this until the water flushes clear. When that happens, you're done! Obviously don't put anything like bleach cubes in the cistern!!! Probably safer not do this in a cistern that's been chemically treated in any way for at least three months (he says, picking arbitrary number out of the air).
To be honest, I wouldn't bother. Fish love the darker water, and your eyes get used to is very quickly. The colours on your fish will look richer. Goldfish aren't wild about low pH levels, so make sure it doesn't go below 6.5. For the plec, as well as most non-livebearer tropicals, pH 6 to 6.5 is just about perfect.
Carbon will remove tannins, up to a point. Depends on how much carbon you use relative to the amount of bogwood. I don't like to use carbon without having special reason: many aquarists seem not to realise it also removes fish medicines.
Cheers,
Neale
