What Levels of Iron are Toxic to Rainbow Fish?

I don't think I ever heard of rainbows being more sensitive to heavy metals than other fish. Since you don't have invertebrates, catfish, loaches, or sheep, I would suggest dosing whatever was recommended by whoever made your fertilizers. I think the goal for iron is about 0.1 ppm. I don't know copper offhand, but it's quite low. There are dosing calculators if you want to work out what will give you the desired result.

I doubt that rainbows are sensitive to iron also. I have well water. There is so much iron in my water that my white laundry eventually yellows or even turns orange. We even have an iron filter set up in conjunction with the water softener. I fill my tank with non softened (and therefore full iron content water!). Rainbows are a group of fish species that actually do exceptionally well in my water. They are healthy, disease free and entirely easy to keep. One of my favorites! Unless you are dosing abnormally high, I really can't imagine that you will have any problems.

wren