If you have enough plants in the tank they will do a good job of absorbing the iron before your fish can be affected, like I said too much or too little and your plants will die off, the too high of levels will take out your fish as well.
Plants are like fish, you need a balance in your tank of certain perameters for everything to be healthy, just need to learn what the plants need, kind of like learning what your fish needed to survive living in an aquarium, its all the same. Remember you have a contained envorment in your tank, since nature can not provide the balance you need to do that for everything in there, that is why I suggested a book, trust me, I spent 5 years murdering plants before I did the right thing and bought a book. Now my plants bloom, the ones that are able to do so, they are greener, stronger, and they seem to bring out the color in all my fish to a new level that I never had in any of my tanks before.