Without a shadow of a doubt fish do not understand the words I speak they do however understand the emotion and energy being projected at them. I am not of the inclination that believes fish to be stupid and possessed of litttle to no intelligence.
I will use a perfect recent point in case. In my 180g sw my now juvi koran angel went on the warpath and my engineer goby was literally being eaten alive as the koran would rip of chunks of flesh. When I came to the net to try to get the engineer I looked at him and told him I was sorry that I had not seen this earlier and that I was going to take him to a better tank. Usually this fish is very hard to net as it hides under the live rock. This time it simply swam to the front corner and curled up and basically swam into the net, no effort. The only movement it made in the net was upward in an effort to get out of the tank as the net moved to the surface.
Chicken skin my adult red devil went nuts in a new tank I had placed him and some other bigger fish in, I caught him and threw him into an isolation 100g tank and left him there for 5 days. The first morning after I had put him in the tank he was constantly in the corner closest to the walk way making sure that he caught my eye. After his 5 days in solitary were up I put him back into the tank with the big boys, not one bit of issue since. I did also tell him that I could cook him and that I did have very sharp knives.
People may think I am whacked and gonzo, but, I have found with all my animals that they respond very well to the respect that I have for them as a life form. Even my kids are totally wigged out at my relationship I have with my animals. Though I do say again I am pretty sure that if I were small enough I would be lunch.