Hi Mark:
It was a mistake that I bought a steel-filled epoxy, and subsequently used it in that photo. I regularly warn others to watch for steel, copper or other fillers in epoxy.
That said, it really doesn't rust in tanks. Epoxy is rather water impermeable and is used in making watertight underwater reapirs. The only thing that could rust is the surface, and I see no signs of that happening. Even if it were to rust, or "leach", it's really only iron at levels so miniscule that there would likely be no effect at all, and tanks are generally low in iron anyway. So, although it was unintentional, I don't think anything bad would happen as a result, and since I have used the better part of half of that tube and see no ill effects at all, I can sort of anecdotally confirm that it is safe in marine aquarai.