This depends a lot on state of mind. When I sell things I do so in good faith, believing that I'm selling them with an accurate description, it's as is where is, and I've been on both sides of it. When I buy used tanks I specifically insist that they be full of room temperature or warmer water (no condensation) when I arrive, or that I will want them to fill it and wait on site before I pay. I offer to do the same to anyone I sell to. Some want to see that, some don't. So long as the OP did not intentionally deceive the seller I'd say he's in the clear. If his conscience requires that he offer the buyer a partial discount, frag, whatever, he is welcome to do so, but that's a courtesy not a requirement. He trusted this tank enough to have it full of water in his home.
Obviously if he knew about the issue and actively hid it that's a separate issue.
Notable cases, all from before I was in the Bay Area:
-A relatively unknown member who sold me (among other things) a broken salinity probe. He assured me it worked, promised we would work something out if it didn't since it was in storage for a while.....and it didn't work when I got home. He offered a partial refund, I agreed, and after several return calls and promises he never made good on it. I don't buy from him anymore. I don't go out of my way to trash his name, but if anyone ever asked me specifically about him I would explain the history. If I saw someone I know well about to buy from him I would probably explain the history as well.
-I sold an ATI light fixture to another member in the local club. I took and then sent him a picture of the fixture with all lights working the morning before he came over. I offered to plug it in while he was there and he declined. It didn't work when he got home. I offered to take a look at it for him (I thought he had an issue with an odd way of controlling it it), but did not offer to take it back. He ended up replacing a ballast on his own. He's bought from me since so I think he was happy in the end.
-A 260 gallon (don't remember exactly, not a common shape) aquarium that a friend sold, as is, where is. Buyer was specifically warned to take the bulkheads out before moving it. Buyer opted instead to rest it on blocks in their trailer without removing the bulkheads. Buyer paid, picked it up, and promptly put it down with the bulkhead resting on the block....crunch. Q: "Do you think it'll be ok (visible large crack)" Answer from my friend: "I'm ok, you already paid, do with it what you want"
-I had one person interested in buying my current reef tank. In addition to the usual scratches there's a giant chip on the back (and internal reinforcement). I absolutely made sure to mention this.