rust is FeO (ferric oxide). i doubt that iron is released as steel 'rusts' (oxidized). i don't have a table of potential energies for oxidation-reduction reactions, so i don't know whether this really happens or not without an electrical stimulus. even if it could occur, only a small amount of iron would precipitate as the surface of the steel is oxidized. once the surface is oxidized, no more redox reaction.
it is true that iron is an essential metal for living organisms. you'll find that liver is the best source for iron in food. not only do algae take it, so do many types of bacteria (including the bad kind). one of the responses of the human body is to absorb iron in the blood stream and store it in the liver when a bacterial/viral infection occurs. in essence, it's a way of starving the bacteria and slowing down the spread. so... i would think that you're feeding a lot of different things if you have any solid iron in the tank, but mostly the bacteria will benefit.