I placed down the aquarium, put in my fish and they seemed happy.
I went to the plaza to shop for some more furniture, came back to my condo and my fish are gone.
They are grayed out in my inventory and can’t seem to find them anywhere in my Underwater Condo.
https://i.ibb.co/1TDG2nb/Untitled-5.png
Can you find them with the item finder tab? They might have somehow “fallen out” of the aquarium, being somewhere adjacent to/underneath it.