Tropical fish despawn from aquarium

Xbox bedrock 1.16 Realms. Usually only 2 players but no more than 3.  Place tropical fish from from buckets into constructed aquarium 8 deep with varying width from 1 to 8 blocks.  Solid columns of water source blocks built with kelp.  Fish stay for a few days of game play with logins/offs but eventually disappear.  Not sure how long the intake time is. Had this happen two times so should be easily reproducible.  Did not use name tags.