Dosing Iron - how long for plants to show improvement?

[vthokie]

I just began to add some iron to my tank, as some of the plants were showing signs of iron deficiency (yellow). How long does it normally take to see some improvement? Days? Weeks?

When things improve, do the same leaves that were yellow turn green again, or is it just the new growth?

I'm trying to figure out how much to dose. I have a test kit, but apparently they are not reliable and most people recommend that you just observe the plants.

I have a 75g low-light tank with anubias, crypts, java fern, java moss and water sprite.

Thanks!