I have heard of the iron nail trick before, for introducing iron to both fish and plants.
Question: does the nail need to be rusting (oxidizing) prior to dropping it in the tank, or will nature just take its course once you put it into the water?
Also, if you have any experience w/ "the nail trick," is it OK to bury it in the gravel, or does it need to be exposed in order to allow the iron to leach out?