If the cat is a few feet away from the tank, it is unlikely the fish will even know it is there. If it is coming right up to the glass, pawing the glass or jumping on top of the tank, it could certainly disturb the fish.
You want to be vigilant when you have a cat and a fish tank in the same house. Most house cats are quite capable of reaching into a tank with their front paws and striking your fish. If you have a cat who tries to hunt your fish, you want to take measures to stop it. Make sure your tank is completely covered, so there is no way for the cat to access the water, and try to prevent the cat from having an easy path to the top of the tank.
I’ve had a cat in our house with several fish tanks running. He couldn’t have cared less about the fish and never bothered any of the tanks, so this isn’t always an issue. Just remember that cats are predators, and if it seems to think of your fish as prey, you may need to take action.