Dolphins in glass aquariums sometimes fall out of the bottom and die also they disappear from tank

They dolphin I put in a aqurium and my son put some in aquarium I put 6 in and 2 fell out the glass bottom other 4 just disappeared my son put a bunch in a tank that had a open hole in top in case it needed air and his disappeared also. Also have had puffer fish fall out of the bottom of the aquarium also and die.