bogwood should not have any effects on the tank. it may release tannins that turm the water a tea colour but this is infact good for fish, it also may soften and help lower pH slightly but loads will be required so we can count that out. the wood may have been dirty as it will have been sat around at the lfs in a tub or on a shelf but a quick rinse will get this off. did you get the wood from your lfs? is it aquatic wood?
if the tank is showing nitrites then i would say its to do with this as its deadly poisonous to fish even in such low amounts. you are doing the right thing by the water change, leave it an hour then retest to see if its all ok again.
how long has the tank been set up? was the tank cycled? if so have you added any recent fish to play with the bioload of the filter causing a mini spike?