Well short term... I'll give you this personal experience.
I ordered a bunch of fish a few weeks back. 2 heatpacks were placed in the box, taped to the top. There were 2 layers of bags. The top layer of bags and the fish inside were fine when they arrived. The bottom layer.. not so much. The temperatures had dropped overnight to a balmy 14 degrees farenheit. The fish on the bottom all appeared dead. Most were belly up, the Bristlenose Pleco
wouldn't move even when poked through the bag. We noticed a bit of movement in a couple of bags so we decided to stick them all into our tanks(fish in bags still) and see what would happen when they warmed up. Eventually nearly everything came around and 3 weeks later is doing fine. One of 10 Otos was dead in the bag. 2 female congo tetras died within 24 hours 1 other female congo took nearly a week before appearing normal. Bristlenose Pleco
, 2 dickfeldi, 6 congos total, 10 otos and some plants composed the bottom layer.
I read another forum post once about a guy who got mad at his cichlid(cant remember exact species) because it had killed another far more expensive cichlid. He proceded to fill a bucket with tap water as cold as he could and put the fish in it for about 5 minutes. The fish survived but from that point on became very passive and docile, likely a sign of nerve/neurological damage.