EDTA? DTPA? Which one do I use?

[dantra]

Folks, like me, with zero/low KH and very acidic water will find that it has an effect on their needs for Fe input and quality of chelate. So my question is, what, if any is the "rule of thumb" pertaining to Fe in reference to soft water and hard water?

Is EDTA better used for soft water?
Is DTPA better used in hard water?

I am looking to place an order so I appreciate any input in ordering the one better suited for my tank.

Thanks
Dan

[jonny_ftm]

And for Tom,

What about EDDHA? Looks promising through a PH range view