Adding it every 2-3 days is about best.
Don't worry if it's 0.0 for a day or two, doesn't matter.
Main thing is to add it regularly.
I use 5mls per 20 gallojns every 2-3 days at the highest normally used
light levels(a bit higher really, 5-6w/gal) to get the best use from a
trace mix.

This works well and doesn't have a testing issue(What is a good
bioavailable level and is the decline in iron due to plant uptake?).
You'll find the answer to the above questions rather difficult to
answer.
Regards,
Tom Barr

"Dave M. Picklyk" wrote in message a...
I got an iron test kit today and found out that I do not have any amounts of
traceable iron in my 48 gal. planted tank. I've added some liquid fertilizer
that is iron-enriched. I've tested the water again in a hour and there is
about .35 to .4 mg/L now...which I've heard was good.

Is liquid fertilizer a good way to supplement iron to a system? How long
will it be till I usually have to add more in? (Or I can continue testing
every 3-4 days for chelated iron levels)

thanx!