Putting the fish in a room and delete it, like I suggested above and was confirmed to be working by OP, doesn’t work (anymore) 
Draw a wall through the fish and they will be deleted. You can also use the same method and press undo, after the wall disappears and the fish appear again it makes them selectable again and can be deleted with the sledge hammer tool.
The larger problem is floating objects in areas with build restrictions, where you aren’t allowed to build walls…
Hi everyone (@here),
- Fish may intermittently appear floating in the air.
This issue was fixed in v0.4.0.
Please create a new thread if you’re still experiencing this issues after the latest updates. 
3 Likes