Zebrafish, when given a choice between an empty chamber in a tank, and a chamber with gravel and plants, preferred the second.
They were then injected with a chemical known to cause pain.
Then, the choice offered to the fish was a chamber with gravel and plants, or, a chamber with painkiller in it. They chose the pain relief.
"One chamber was barren and the other chamber was enriched with gravel, a plant, and through the transparent rear of the chamber a group of zebrafish provided social enrichment. Zebrafish displayed a clear preference for the enriched chamber for at least six consecutive tests."
"At this point the zebrafish were injected with saline, a non-noxious control, or a noxious chemical and placed in the maze. In half of these trials, an analgesic was placed into the barren, unfavourable chamber."
"Those fish experiencing pain lost their preference for the enriched chamber and were willing to spend more time in the barren, unfavourable area only if analgesia was present."
"This demonstrates that the fish were willing to pay the cost of being in an unpreferred environment to obtain analgesia, and thus it can be inferred that these fish must have obtained some reward possibly in terms of pain relief such that the pain was reduced."