Welcome to the forum, SueL. Planting cattails was a great idea. Cattails are an excellent wetland species and will help "clean" the runoff to your pond.

The first thing you want to ensure is the iron in the runoff is not from rubbish. Walk the watershed and clean it of any debris, including ferrous metals.

Second, you need to slow that water down before it enters your pond and as far away as possible. Metals and most things detrimental to water quality are heavier than the water. Slowing the water will help precipitate the minerals away from your pond.

Last, and one of the most difficult is a natural filtration barrier. I use a combination of rip rap (large rocks), river rock and sand. This also requires a sand stop if you don't want the sand entering your pond.

There are a thousand things you can do and hopefully others on this forum will pop in with more ideas.