If you mixed your DNA with a fishes DNA can you turn into a mermaid and then turn back into your errenalal self?

No. Fish DNA is different than human and a person can not survive if that happened. Think about it does it make sense that a person can be part fish and part human ? Why not part cat or dog? If you think about this logically you know that people can not become another thing or creature.Mermaids are not real as well, so you want to be something that doesn't exist. Think. If you want to pretend you are a mermaid go buy a costume.