Even if they sedate the fish before hand, when they come to they have to heal. Healing wounds tends to be painful (I believe fish feel pain, this is why they try to get away when another fish is eating them). There is a high mortality rate, and many secondary infections.
I can get over this, after all I eat many animals and enjoy doing so.
I have never seen a tattooed fish that doesn't look like random jabs with a pencil though. The worst homemade Slayer tattoos tend to have cleaner lines, which leads me to doubt the sedation. It just looks bad, maybe they could use some Sailor Jerry flash art or something and make it last more than a month, and I wouldn't hate it quite so much.