Fish randomly disappearing

I have an aquarium that is 2x4x2 in volume. With water and carol on it. Two days ago I added 3 tropical fishes into it. Today, the fishes all disappeared. They didn't left any loot or experience orbs, so I don't think is dead. This happens every time.

I don't know if the picture or painting is doing something.

I am in singleplayer.

The following pictures shows before of the aquarium and after when the fishs are gone.

Note: Yesterday night before i paused the game, the fishes are still there. But this morning when i resumed the game, the fishes are gone.

 

Thanks