Fish are swimming through Granite Tiles which appear to also contain water.

When tiles show lit like that it means they don't "exist" anymore. If it is a sandbox save file, you probably painted gas/liquid there. If it is not sandbox, then tiles broke but for some reason stayed there. Reloading usually fixes the issue

It's not sandbox but I ended up rebuilding the tiles which fixed it. I will try reloading next time but I am pretty sure it was persisting through a few save/loads before i figured out what was going on. 

Editing to add that I noticed that it happened again to the rebuilt tile. Save/load did fix it but I am guessing it will happen again.

Edited Wednesday at 07:21 PM by Chuckw56
Adding more information

As an update, this continued to happen until I replaced the tiles with airflow tiles.