Totally incorrect my friend, but thanks for bringing it up! Fish do seem to feel true pain. I don't know how this misconception came about (and it is a common one, even in science), but yes, like us they have a nervous system and a brain, and detect when their body is being damaged and seem to receive that stimuli as pain, just as we do. they also have reflexes (like us) and nociception too, and additionally they seem to feel pain as well as any animal. Fish also possess pain nerve fiber tracts in their bodies that are similar to those that tetrapods, leaning towards them certainly being built to feel pain.

Some people confuse the issue by claiming (with very little proof) that only creatures with a neocortex as part of their brain can feel pain. This seems to be the root of the misconception, and it is wrong (because animals without a neocortex like dogs and cats do certainly feel pain)

I have heard a few good theories towards fish not feeling pain, such a the theory of a reflex (like when we burn our hand and our spine automatically removes it from the danger, even before our brain detects the danger) but given that fish actively avoid things that have harmed them in the past, this goes beyond reflex (because, for avoidance there had to be a stimuli, good or bad, in this case pain and it was bad, so they avoid that situation again. We avoid the stove again because our brain later registers the pain, not because of our reflex action. If it was just reflex we would not feel it and would not learn to avoid it, after all.)

In fact, it's only been recently that studies have been coming out that confirms what most fishkeepers know, that fish don't just act on reflex, they learn from being harmed to avoid whatever harmed them again, as a result of being in pain. They will also nurse injured limbs (another sign of registering pain) and 'flash' and rub against gravel when infected with parasites on the skin/scales (so, either an itch or pain, both are interconnected anyway)

There was an interesting study recently that seemed to prove this once and for all. One group of fish was shot up with morphine, and another with saline placebo. Both groups were then burnt/forced to endure burning sensations, and both set of fish showed the expected reflex behaviour (the same we do with the hot stove, a non pain behaviour. Keep in mind these are separate behaviours, reflex vs pain). HOWEVER, after, the morphine fish shown no signs of pain and continued to act normal (until the morphine wore off), while the saline/placebo fish acted withdrawn and what classicial fishkeepers would identify with their fish having received an injury/being in pain (nursing sore site, etc). this also seems to rule out nociception being the cause of pain like behaviours, ie, it is true pain. Since then theres been a heap of studies along this line that support our fishy friends feeling pain. Additionally, while pain receptors are generally not found in cartilaginous fish (sharks, rays, but that doesn't necessarily mean they don't feel pain) they ARE round in bony fish, like the ones we keep in the hobby. 22 of the 58 sense receptors found in a trout's head, for example, are known pain receptors. Interestingly, the poor trout in this study also shown post-traumatic stress reactions like rocking and other abnormal behaviour after their unfortunate experience. Theres a good Dr Karl article on this one somewhere if you're interested, and I'm sure I can find the article with the morphine fish too, if you give me a moment. Theres also a BUNCH of other articles like this out there

We still don't fully understand pain in humans, but yeah, fish feel and react to pain easily it seems, which goes against what was traditionally though (then again, lots of traditional thoughts are wrong and are replaced by newer, better proven ones). They also react to uncomfortable temperatures. Pain is a pretty old thing in the evolutionary chain, it turns out.

Pain in trout . Morphine fish

Therefore, since it is most likely (and I certainly do believe, based on my experience and the current research) that the fish under my care are capable of feeling true pain, it is paramount that I make everything, including death if I must bring it about, as painless and as easy as possible for them. :)

The more interesting debate is how fish react emotionally to pain. Neurological pain happens to us all, including fish. It's the capacity for suffering that remains up for dispute, ie, they feel the pain, but how does it affect them long term, if at all beyond learning to avoid something because it causes pain? Theses some interesting evidence they have a rudimentary (at least) emotional reaction that I mentioned above with the trout to traumas and pain (And I have known fish, especially intelligent ones like oscars, to 'sulk' after they injure themselves somehow) so theres probably even some emotional capacity for pain there. I can vouch that goldfish, for example, often pine when their long term tank mates are moved, particularly if it was one they got along with well, so I do think they have emotional capacity for many things. They may not understand and it may be more muted then animals with more developed brains, but it's there.