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When iron (Fe) is deficient, plants will produce less chlorophyll in their new shoots. An iron deficiency is therefore easily identified in the shoot tips of fast-growing stem plants. The rich leaf green fades, and the young plant parts assume a yellow to white colour (chlorosis).
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So having Fe in abundance (assuming no other deficiency's) the plants will be a the colour they should be if it is red or green will be plant dependant, adding extra Fe will not make them redder.
To make some plants redder you can eg reduce NO3 levels which initiates a NO3 deficiency which makes some plants redder- but it doesn't mean they are healthier.
The spectrum of your light output can make the tank look better also, much of this is down to what the user likes, but ADA Solar RGB really make the reds pop - seeing is believing.
So picking the right plants and having the right light is the best way to get the reds to pop. If you use a sodium light plants will grow fine but look washed out and pale and reds will be very poor.
So in short, adding extra Fe when Fe is already in abundance will change nothing