Wouldn't a nail rust in the tank? Or give off harmful minerals? Is there an alternative? Maybe stud screws?
I would have thought that odd bit of rust that might come off a nail in a fish tank would have no harmful effects. If you worried about such a small thing imagine the nasty things you put in the tank when you put your hand in it and yet the fish survive still.
Well rust is iron if I'm right. Is it toxic to fish?
We are talking extremely low levels of dissolved iron here, there is a lot more in any aquarium plant nutrient
than would come out of a rusty nail. I have two pieces of wood held together with two screws in my larger tank, they have been there for two years with no apparent problems. Your water is in contact with the metal in the impeller in your filter (although not wholly iron there will be an iron content as it is magnetic). I really wouldn't worry about one or two nails.
good suggestion 
thanks!
It means that the wood gets to sit in water that is renewed everytime you flush your toilet. This saves you having to change the water in a bucket etc. Providing it fits and won't interfere with the working then it is a decent way to do it. Especially as you have already poured boiling water over it to remove alot of the tannins.
