I'm not sure if anybody mentioned this, but in California (and maybe other states) you only have to pay what you are first quoted. I know this first-hand because I worked at a smoothie store that had merchandise, and a customer asked me what a mug costed, and I said, "Um probably around $3.00 but I'll ask my manager". When he came out, he said to me,"Well we have to sell it for $3 now because that's the state law in CA." The actual price was closer to $5, and we ate the loss.
So, if the sale occurred in CA or another state with the law, I would say keep it. If it didn't, I'd say pay up to the %50 price, as was mentioned above, so that you can keep a decent relationship with the store (you obviously like it there, since you went twice within a week and its 3 hours away). Plus, if you are the type to feel guilty, then you should make at least a small concession.
What I think is weird was that they asked you to return it and then they would give you the right fish... thats just dumb, because the "right" fish is the one you have (that you wanted), but for the "wrong" price.