My friends mom had 6 cats, and two fish tanks; one goldfish/pleco tank, one betta/pleco tank. Every cat ignored the tank, except for two of them, Pebbles and Gizmo.
Pebbles would sit on top of the betta tank (a 30 gallon hexagonal tank) and paw the water. The betta would actually play with her,coming out but staying JUST out of reach of her paw, inching closer, and then swimming out of reach again. Quite funny to watch the interaction.
Gizmo loved her goldfish. I used to do maintenance on the tanks for her (she had no idea what she was doing, she told me once that the cats were her pets, and the fish were her cats pets). Gizmo was a rescue cat, and was exceptionally skittish and afraid of everyone. Regardless, she would curl up at a safe distance and watch my every move while I tended to her friends in the goldfish tank. The one day, I was changing the substrate in the tank so I had the goldfish and pleco in a 7 gallon bucket. I left the room to grab the second bucket for gravel, keeping the tank in my sight. Gizmo searched the tank frantically for her friends after i left, and then found them in the bucket. She just sat and watched them, didn't even touch them. When I walked back in, she went back to her watching spot, I finished up the tank, and as I was heading out the door, she was sat, paws up against the front of the tank, meowing at her fish friends who were watching her right back

. Super cute!