I think if Fe were the problem it would not be coming from the new tank. But what would small amounts of Fe in the water do? I worked for 15 years around GE boiling water reactors. The reactor used “pure water”. However, even the smallest impurities concentrate over time when the water is boiled away. So the design included a reactor water cleanup system.
Yes, any fouling of surfaces interferes with heat transfer.
Whatever, the cause of the crud, I think the Marathon with no water exchange (and algaecide) would eliminate it. No Fe source from a tank and no other sources of Fe or other stuff if the Marathon is a holding tank of stagnant water.
You would want to vent the Marathon to atmosphere. So consider the need to possibly add a little water every so often due to minimal evaporation. If you build a loop seal in the vent line, condensation should fill it and stop “boil off”. The final vent could have a cap or pipe plug with only a 1/16 inch hole in it. This would take evaporation down to almost nothing.
If you do this project, post again and let us know the results. The proof of the pudding is always in the eating!