And you forgot Fe-HBED. ;-} It's a fruit-punch variant in that it will color your water red, but the red is much less intense than Fe-EDDHA. And it's the most UV-stable chelate.....

There is one other choice Stuart. Let's not forget that regular iron like what you find in Maxicrop plus Iron works depending on the pH. Just because it's regular iron doesn't mean you aren't supplementing what's in the system.

I thought the whole point of adding iron in these forms was because it was available. Otherwise we would just throw a horse shoe or two in.

By normal I mean bio-available Ferrous iron although iron really is in a lot of forms. At pH's over 7 it's less likely to be in this soluble form. There's no guarantee that you'll find it in a bioavailable form at lower pH's either. There are other possible supplements for Iron as well.

It's also possible to have plenty of Iron in the system but not have enough available when your pH is in the right range. I was reading this article - http://www.skepticalaquarist.com/ferrous-ferric and toward the end he suggests adding organic chelating agents instead of Iron (Scroll to the last paragraph unless you want to read it all). Basically you'd be chelating the Iron that's already present in your system and making some of it available that isn't. Think of all the Iron you've added, did it all get used by the plants? Probably not.

So how many horseshoes per 1000L should we be adding?