Krakaboom
aka Justin is a 37.33 year old boy, has been a member since December 3, 2006, has scored 814 submissions, giving an average score of 2.51, helping 72 designs get printed.
This happens due to motor feedback which doesn't require the brain to trigger it, probably because it's something the fish does all the time. It's dead, it just keeps moving.
A similar thing happens when a lizard drops its tail. electrical impulses keep the muscles twitching, which makes the detached tail wriggle, distracting predators while the lizard escapes.
Nice little explanation from one of the YouTube commenters:
If you used salt on them or lemon you activated the sodium potasium pump for muscles.
Muscles work on contraction by a sodium potassium pumps in the nerves, Considering this fish is most likely fresh the proteins acting as the pumps didn't denature/ die off. As such when you placed salt or lemon on top it activated the pump. It basically tries to keep the concentration of sodium on one side and potassium on the other in equilibrium. Since you added salt (NaCl) it activated the pump (Na+), enjoy