when you say iron tabs.. do you mean flourish tabs from Seachem and Root tabs from Aquarium Pharmeceuticals?
if so, then I would still add them. I noticed that some deficiencies show up without the tabs. The thing is that the tabs don't just provide iron, but all the other micronutrients as well.
some plants like the apon bulbs, or crypts would definitely benefit from added substrate fertilizers b/c they are such heavy root feeders. For apon bulbs, if the substrate ferts are not provided, it may end up using its stored "energy" in its bulb and the bulb will eventually rot.
I think that 1/3 or 1/2 a piece of Jobes 13-4-5 sticks would be a great addition to the aponogeton madagascariensis.