Among sellers, this is controversial. Some feel that if you didn't buy insurance, you're out of luck. Some feel that you didn't bid on a dented tank, so it's the sellers fault. In truth, insurance isn't for the buyer, it really protects the seller. (think about it...)
Both the seller and the buyer are inexperienced- and it shows. he has less than 200 sales, yet 5 negative feedbacks. He takes small, dark pictures that don't show anything. Unless somthing is weird about his perspective, i don't think that would be an oversized box. Yet- you bid and won.
I would not have done business with this guy.
So, I would tell him that his packing was insufficient and you would like to return the tank (at your expense) for a refund. If he refuses, you can try taking recourse through Paypal, if you paid that way, or through Ebay.
If those channels don't work, leave negative feedback ( bad packing. Came damaged) (don't attack him- it makes YOU look bad) and think a little harder next time. For instance, if the seller has crummy pictures, ask him to send you beter pix via personal e-mail.