well, what kind of battery was it?

1) if it was an alkaline, it may not be that bad
2) you would be suprised what salwater can chemically neutralize - nasty stuff like sarin and mustard gas for one. (my sister used to dismantel chemical weapons and destroy them for a living... one of their fallback safety features was 'da ocean' ;)
3) if the batteries casing did not rust through, your probably looking at a little excess iron, and maybe not much else. I doubt the copper that remains is to bad, because, we know for one you have no problem w/ invertebrates living in your tank... peps, aiptasia, tons of coral...

I don't think your as bad off as you think, biological indicators infact, are pointing to life thriving in your tank...