fish, specifically goldfish have a 3 second memory
fish don't feel pain
fish only grow to their enviroment
fish have a highly evolved memory and will remember things for years, take salmon that remember exactly which river they were born in and from 1000's of miles away remember how to get there.
fish do indeed feel pain; how they interprate those sensations is in debate but they do feel it.
certain cryprinid species can slow their body growth down, to a level that from our perspective looks like the have stopped growing, but they can not stop growing. they can not stop their internal organ from growing at all and this often results in disformed fish and an early death. Stunting as it is called.