I always hear .1 ppm mentioned as the standard desirable level, however you'll have a tough time finding any test kit that can measure Fe levels that low reliably. I've read that Fe levels in excess of .1 ppm may provide greater benefits for many plants, but one has to be careful if dosing it heavily with fish in the tank as it can be toxic at high levels. I find the best indicator of sufficient iron levels to be the coloration of plants like Didiplis diandra and Pogostemon stellatus. Hope this helps
You don't really need to test for iron in your tank as long as you don't overdose. I really wouldn't worry about it. I dose .1 ppm every other day in my tank and have had good luck. If you do feel the need to get a test kit, here's a link to a good one, although I'm not sure it will measure levels down to .1 ppm. Kind of pricey too :wink:
Turtlehead... Keep a eye on your plants they will tell you when you need to add iron. I add iron once a week to my tanks, 1ml per 10 gallons. You can start out with this amount and if your plants start yellowing you can add more during the week. I would not waste mone on a iron test kit, I had one that gave me a 0 reading no matter how much I added.