If you purchased them yesterday and had them in your tank yesterday I think you should really rethink your quarantine procedure. I quarantine any and all plants for one week minimum. During that time they get PP dips (bleach dips if the plant can handle it) and then 3 day soak in alum. I do this more for killing off critters of course but it should handle any strange bacteria as well. It could have been fertilizers, copper, bacteria, a number of things but yes, if that's all you did and had that many deaths I would surely point to the plants. As far as 'iron band' if you are talking about the plant sinkers they are zinc I believe and I've used them in shrimp tanks long term with no ill effects noted.
As a side note, I have stopped adding any plants to any tanks that are important to me. I have a tank full of cherries and crays that I will sometimes put something like a moss ball in to test. But my two other tanks with more expensive shrimp in them, I simply am too paranoid to add any plants. They are stuck with what I start the tanks with