I would be curious as to the validity of such an approach as most of the time, iron bonds to other minerals and becomes unusable to the plants. Alternatively, you can buy some cheap fired-clay aquatic soil at most pond stores, the home depot here sell it for about $5, and mix it in with the gravel. Kinda ugly though. I've used plain old potting soil made for cacti and palms that was supposed to be very "loose" underneath gravel, with the theory that it wouldn't stagnate. I got tired of the tank after about six months, but I had absolutely no problems and no algae blooms or nutrient spikes. It didn't smell or anything when I took it out. I did get some really funky stuff growing out of it, namely two different aquatic worms, a bunch of niterite snails and some cool stringy-looking bladderworts. This may not me the norm though, I am just trying to help.