I don't know what ebay's rules are on this, but I would immediately contact the seller and say that it was his/her obligation to pack the item with due care and get it delivered to you in good condition. After all, you paid for an intact fish tank, and didn't get one. S/he'll argue that you should've insured it (and as a practical matter that's true), but nonetheless, through no fault of your own, you didn't get what you paid for.

Unless you want to track down the seller and sue them, it mostly comes down to how accommodating they are and how much they want to avoid negative feedback. You should also check ebay's policies to see if they can help you - I think they have a mediation service or something. But in the end they probably can't force the seller to do anything - they don't normally get involved in legal disputes.

The lesson: always insure packages. I require it even when I'm the seller, and say so on all of my listings.

Good luck.