iron is in fact one of the few metals that's fairly harmless to fish.
there are many rivers where the rocks and sand are visually 'red' due to huge amounts of ferrous oxide in the water and the soil.
In the old days we used to have metal welded tanks with glass windows (!), often such a tank had a fairly rusty bottom......and you had to stand next to it with a machete to prune it!
I used to dump a handful of blun ted nails in the filter or on the tankfloor, before the sand went in; that's back in the days when a 3-4" fish was really big.
the small Cory's and loaches etc couldn't get to those nails, but the iron sure got in the water!
adding iron is THE way to keep red-leafed plantspecies thriving and also for healthy Echinodorus growth it is a MUST.
I also used solid ferro-fertilizer; reddish crumbs I had to put deep in the sand. got loads of experience with plantgrowth. decades.....
About aquarium fertilizers there are a few things you should know:
-first; they are harmless for your fish! stuff bought at an LFS is good stuff, usually.
but do NOT use the liquid fertilizer you can use for room plants (!!!). that stuff contains loads of phosphates, nitrates and even ammonium; you kill your tank.
-second: the dosage written on the bottle is
always way too much. it is written there to make you buy a lot of it, to make your money go to their wallets!
use a quarter of the dosage or even less.
-third: patience, young Padawan!
a fertilized plant doesn't show proper growth in less than a month, and red plants don't show proper growth or deepened colors fast either.....if you think after two weeks "it doesn't work, let's add another dose", you're going to need algae-eaters pretty soon.....
-fourth; it goes without saying that all the absorbing filtermaterials are a no-go; activated carbon, zeolite , aquasafe or similar tonics and easylife
all neutrilize and bind aquatic fertilizers and spore elements.....so if you have this in the filter, get it out or waste your money and efforts.
good luck with the plants.