Superflat World; I built three aquariums and a pond (with a glass top due to fish jumping out of the pond), in the previous Snapshot, Minecraft 18w21b.

Two aquariums were connected with an awesome tunnel for the fish, it had all fish mobs except pufferfish.

One aquarium was topless and had all fish mobs and dolphins.

The pond just had pufferfish and tropical fish.

However when I played this world in Minecraft 18w22a, All my fish had disappeared. My Dolphins were still there, that is, until I paused the game and exited out of full-screen and returned a few moments later, that my Dolphins had disappeared. I'm super confused as what happened or what went wrong. 

I pressed F3 and A to Reload chunks, nothing happened. I closed and reopened my world to see if it would correct itself, but it didn't. My World went from day and clear to night and rainy.

I have attached screenshots from 18w22a of my empty aquariums, pond, and debug/reloading chunks result, as well as the reopened world results. 

[Edited:] I went to try to replicate this and recorded the gameplay with my phone, I edited it using Spark Adobe, snazzy music called Building Tall provided by Spark Adobe itself. 

So I opened a new world in 18w21b, made 3 different aquariums, one with all fish mobs except pufferfish, one with all fish mobs and dolphins, and one with just pufferfish. Everything explained in video. I recorded everything on my phone, because unfortunately my game recorder has stopped working completely. 

The fish and 3 of 4 dolphins happen to still be there, in both 18w21b and 18w22a. I'm still confused as how it just one world it happens and another it doesn't.

I uploaded the video separately unto YouTube, not my usual YT account that I usually use, but it was an emergency.

Unedited version: https://youtu.be/WACplcEbnmw

Edited/Short Version: https://youtu.be/9f5ObXX7skM

UPDATE: All right now, I now know this is a bug that's still going on and not a one time glitch. I haven't played on previous pre-releases but on 1.13 pre-release 5, I had bigger aquariums and had all fish and Dolphins. While I was about maybe 100 blocks away, I was doing a pufferfish rave similar to Grian's, from a building tips and tricks video showcasing 1.13, and I came back and found that they disapearred, Just like the snapshot one. Someone fix this please. I Debugged this world 4 times already. 

[7/2/2018 12:19PM CST]