Really, all animals are individuals. All the cats & dogs I've had never showed much interest in fish. Except one dog that I had to bring back the day after adopting him... he was so focused on the 75g tank, jumping up at it, trying to get behind it, staring & lunging. I was too concerned that he would "discover" the small betta tank or the dart frog tank, either of which he could have knocked over with one sudden jump. I've never heard of a dog having that kind of drive towards fish... and I worked in an animal shelter for 14+yrs. So there may be outliers, but most cats (& dogs) will be perfectly fine with a covered tank. Even enthusiastic cats at least can't knock a decent sized tank over.