Fish Dissepeared when I looked at them

Still dont know why it wont let me put 1.16.4 On affects version

This is a visual bug and can be created by mods such as Optifine! Make sure you are using the vanilla client. Also, in the future please elaborate further such as on how to replicate this bug!

unable to replicate, please provide more details. 

I dont have optifine on its vanilla minecraft 

If I also was on optifine it would be smooth but I wasnt on optifine cause it wasnt smooth and if I was on optifine also optifine isnt a mod I have optifine installed but not on forge this is just normal vanilla minecraft

if you dont believe me I would use my snipping tool I dont know the font for the top left so I cannot photoshop it and I will edit a screenshot on this and tell you I wasnt using optifine codexe

edit as in like put another screenshot on the bug

Sounds like MC-197328. Does that issue describe yours?
Also, old versions are not supported, so please only report bugs for the latest release and/or snapshot.

Thanks!

Yellow Crewmate, I was just confirming as you didn't provide a debug screen. When you are making these reports, please open your debug screen when taking the screenshot. 

Anyways, I was unable to replicate it, but judging by your image, the fish just swam into unloaded chunks (I can see a bit cut off in the back). Again, attempt to replicate this with video footage.

Yep, this ticket is definitely a duplicate of MC-197328.