My Uncle Ray used to use Sears sump pumps to run fresh seawater into his live bait shrimp tanks. He just dropped the sump pump into the ocean near his bait shop. When the plankton layer rose and fell, it would get sucked into the sump pump a couple times a day, and you could actually hear the sudden clicking of all the shrimps' claws trying to grab it. Some bits would survive the impeller and the shrimp attention and survive. If an egg or larvae grew into a fish, Uncle Ray would leave it alone, unless it started to nibble on the shrimp. Then he would catch it out and place it in the"creek" of saltwater running from his bait shop back to the Atlantic. One time when I visited him in south Georgia I saw a nice lookdown that had "appeared" and grown. Every few months the pump would turn into a rust ball and stop pumping and Uncle Ray headed back to Sears for another. His system was open (=constantly new water replacing the old). Even in a closed system I don't think the iron would be a problem, but as was said, any copper would be really bad for our closed systems. And who knows what other materials are present. I would sure do some research before buying a pond or freshwater pump for marine use.