I've looked under fish tank and aquarium without seeing this.
I am assuming this is a glitch. When I went to buy a tank it said north or east facing. I bought a east facing deed and when I went to set it up it was north(south) facing.
In looking into this I've seen that it was reported that removed fish did not get removed, is that still a problem? Don't want to test it out by removing one of my fish and thus exploit.
I wanted to look further into fixing this and assume discussing that should be done in code talk or feature requests, however I'm more looking to get information to use to fix it and submit.