I used some nickel plated magnets to hold a riparium planter in a tank for about a year and a half or so. They corroded a little bit, but not much. Magnets don't attract iron ions in the water, nor do they attract red iron rust. They will not affect the chelated iron in the tank. I doubt that the corrosion on the magnets will do any harm, but as noted above you can buy vinyl coated magnets to use. Ebay usually has someone selling them.