Name Tags Don't work correctly

I was making a fish tank in creative, I generated several fish of different species (tropical fish, cod and salmon), and I put a label on all of them with the name "Fish". They were in their well-lit fish tank to prevent the generation of axolotls, and they were fine for about 2 or 3 days, but today they disappeared. The labels serve so that a mob does not disappear, right?