ive got 2 cats and got a tank of fish a few months ago. at first cats seemed pretty interested and would sit infront of tank, but novelty of it was gone after a few days. although they do like to roll infront of tank sometimes. rather odd to me!!!
i've had 10 cats during the life of my goldfish (the fucker is immortal or something) and they've all had very differant reactions
my parents cats were so determined to eat him he had to live in the garage and one cat in paticular was so obsessed with getting at him he'd lurk by the garge door and streak in whenever someone opened it to try and push the fish tank off the counter.
my first cat spent hours every day tapping at the tank and did occasionally try and shove the tank off the mantelpice but never succeeded
the next cat never showed tha faintest interest although ironically he was the only one to succeed in knocking the tank down and almost murdered the fish, he jumped on it one day and slipped and dragged the tank down with him from its windowsill. fish had to live in a jug for 2 days til we could get a new tank.
that cats mother lived with us for a while but only ever looked up at fish from floor, showed no interest in chasing.
current cat has been sitting right next to the tank and not noticed fish and her kittens are 2 small to jump on the counter but i'd imagine they'll be interested judging by the fact that they chase everything else that moves!
when i was a kid my dad used have a huge aquarium and we'd put a chair in front of it for the cats to sit on while fishing. they'd be so busy frantically jumping up at the glass at the fish they're invariably fall off the chair and look very embaressed
just make sure the tank has a lid and is located somewhere they can't get behind it to push it (cats asre cunning enough to think of shoving it over a ledge) and fish will be fine