The tanins released from wood are, in fact, quite harmless to tropical fish. Many species actually enjoy life better in slightly coloured water, as it is closer to their natural conditions. The only thing you will have to worry about is the redution of light getting to your plants, but even then, this reduction is absolutely tiny.
If youre running a tropical tank, be careful that the wood doesnt reduce the pH of your water too much.
Also, the colouration will be heaviest at first. Its nearly impossible to get all the colour out of a piece of wood, but in a few months time the tanins will be released much slower. You will only have to do more frequent water changes if either:
a) your pH becomes too low or too unstable
B) you like having crystal clear water.
Its worth mentioning that many people actually buy tanins in bottles to add to their water because they like the colour better. I personally like it a lot, its a far more natural environment. After all, how many tropical ponds or streams would you expect ti find absolutely crystal clear water in, considering the amount of dead leaves and bits of wood would be in there?
Also, charcoal will only work to a limited level. The only way to get rid of the tanins is, basically, to replace the water.