- How big is your tank
- Heavy, moderated or low Plant mass
- Light and light period.
And Yep rusty nails (if not contaminated) will procured the ferric form of fe (Fe III). Less accessible for plant compared to the ferrous form(Fe II) but can be recombined with chelated agent like Humic acid, fulvic acid and tannic acid are examples on non-contaminant organic complexes. Phosphate also serves as a very effective complexing agent for iron, becoming available again to plant in a somewhat less soluble form than Fe II ferrous.
OXIDATION; (Ferrous) available to plant wen oxidize to become (Ferric)
This an idea for the time it take for Fe2 to oxidized:
At pH 7.0,
90% Fe2 oxidation requires 1 hour at 21 degrees C and
10 hours at 5 degrees C.
At pH 8.0, 90% Fe2 oxidation occurs in 30 seconds,
at pH 6.0 it requires 100 hours.
The critical dissolved oxygen concentration is 2 mg/L. Below that ferrous iron oxidation occurs slowly.