Iron quickly degrades in the water column unless it is chelated by EDTA or DTPA.
Toxicity depends on whether the iron is chelated or not. Chelated iron is less toxic than unchelated iron. Unchelated iron (FeCl3) becomes toxic when the concentration in the water is maintained at around 1.1 ppm for at least a week. Chelated iron becomes toxic at some value above that. I do not know the exact ppm value for chelated iron that is toxic, but after adding about 1.7 ppm DTPA chelated iron to my tank I noticed some black spot algae turned red and died (a sign of stress) and a few R. macrandra leaves curled a bit. See thread:http://www.aquaticplantcentral.com/f...tml#post664786
Iron toxicity signs in plants look a lot like iron deficiency signs - pale/white new growth. This is because iron is not mobile within the plant, so in a deficiency situation the iron cannot be removed from old growth leading to pale new growth. Iron toxicity inhibits the use of sulfur in plants leading to a sulfur deficiency. Sulfur deficiency looks very similar to iron deficiency (pale new growth), so iron toxicities and deficiencies often look the same.
I'd recommend keeping between 0.1-0.75 ppm iron in an average EI tank.
Unfortunately iron is difficult to test for. Of the 5 major forms of iron people dose (citrate chelated, ferrous gluconate, EDTA, DTPA, and unchelated iron), only EDTA and DTPA chelated iron will be detectable in test kits. If you dose the other types of iron you will not see any measurable amount in the water even shortly after adding it to the tank.
There are a few different theories on what type of iron is "best" for plants. Citrate and ferrous gluconate iron are thought to be more easily available to plants since plants actually form citrate and ferrous gluconate inside plant tissues for transport purposes, so the theory goes that plants will more easily use iron if it is already in the form they use internally. These compounds very weakly chelate iron and are not very stably bound to the iron, this is probably why iron levels are not detectable in tank water shortly after dosing them (the bonds break and then iron oxidizes to an unusable form Fe+3). EDTA and DTPA form tighter bonds with iron and keep it in the Fe2+ state (the state that plants can use) for long periods of time, so they ensure plants have access to iron for days or even weeks. DTPA is the most tightly bound chelating agent for iron, so it persists in the water column for the longest time. The thought on EDTA/DTPA is that while the iron is kept in usable Fe2+form for long periods of time, plants must expend more energy to break the [EDTA/DTPA -- Fe] bond in order to use the iron, which makes using this form of iron more energetically costly. Finally, unchelated iron is easily degraded because it is unchelated and vulnerable to oxidation in the water, and also more toxic in lower concentrations, so most people don't use unchelated iron.
Which form is best?
I suppose you could hedge your bets and dose a little bit of either citrate or gluconate and use EDTA or DTPA iron for long term availability. Personally I prefer EDTA or DTPA iron only since they are the least toxic and longest lived forms of iron. But I suppose that is just a personal preference. I'm sure you'll find people who love citrate and gluconate iron over EDTA/DTPA.