It's a macronutrient deficiency, not iron, probably phosphorus judging by what you've told me. When people do a fishless cycle with plants, they add ammonia to feed the bacteria, which supplies nitrogen, but without fish there's no source of phosphorus. Once you add fish, there will be phosphorus coming into the tank via fish food (and subsequently fish poo) that should mostly satiate the plants. Unfortunately, the affected leaves are probably toast at this point, but you'll regrow more as the plant recovers. You should also be dosing a micronutrient fertilizer, which you just started, but that's not playing into the issue you're experiencing.
In an ironic twist, if you HAD been dosing ferts from the beginning, especially Excel, your problem would be much worse as the demand for phosphorus would have been increased. What an odd world we live in eh?