has anyone used this as a medium within a fish tank?

I have never heard of it...........sorry

Not heard of it either. What is it exactly?

Just did a little research...

Hydroton, a.k.a. hydrocorn, are kiln-fired clay pebbles imported from Holland. Their neutral pH and excellent capillary action qualities make them a great hydroponic medium. Preferred by horticulturalists for hydroculture, this product offers many advantages when compared to soil. The medium is clean, odor-free, reusable, and light so as not to crush plants. It offers excellent aeration. Hydrocorn prevents rotting, excess acidity, and soil pests.

it floats ! no worries for a hydro setup but no good for a substrate!

cheers

I've used it in my hydroponic setup. When soaked for a long time it no longer floats.

It's not a bad idea actaully to use it as a layer in the substrate. Probably has the same effect as laterite.