My daughter has an aquarium created for dolphins.  It goes fully down to the bottom of the river she has them in.  It is 10 blocks wide, roughly 15 blocks deep and several blocks above the water.  Twice she took dolphins from the ocean on lead and brought them to her aquarium.  She named them and exactly two minecraft days later they despawned (no cod left behind).

To try again, she used creative mode, spawned them in the tank, named them, went back to survival and exactly two days later I went back.  I see one dolphin, but not the other.  I enter the enclosure and in that amount of time, putting it exactly at two Minecraft days, the second despawns.