You may be thinking this is not a good idea! For heaven's sake, cats are natural born fishers! Well, yes, but cats, in the first place, can, yes they can - be trained! To ensure the safety of your fish, though, there are plans you can put in place to make their habitat a safe one.
Train your cat?! There is much written on how to train your dog - but the family cat? Believe it or not, it has been done and so is possible. It requires a lot of patience, though. You can teach your cat to "sit" and "stay" and "Lie down", among other tricks to keep them from attacking your fish. It goes without saying that we must also take very good care of our kittens and cats and give them what they need to be healthy and happy so that they will not act out. Be their best friend and they will be yours!
My cat, Sandy, actually REMINDS me to change my Betta fishes' water every 2 days! Oh, yes - he sits on top of the box where I keep the fishy water change supplies and looks at me and meows! The fish were there before he was, so he got used to them and the water change process from the time he was a tiny kitten. Now, he's a big kitten, but he is accustomed to the routine and doesn't even really take a great interest while I am changing the fishes' water, but he just wants to make sure that it gets done. He seems to sense when his fishy "big brothers" are at their comfort threshold and need clean water to just keep swimming and just keep swimming around in their happy-go-lucky fashion!
On the other hand, just to be sure - have your fish in a living space that is safe so no curious paws swish around in the water! An aquarium is always the best choice for your fishes' safety and well-being. Besides supporting better water quality to ensure a longer, happier life for your fishies, a tank comes with a lid! This, of course, can be great protection from the outside world - including kitty's cute little paws and claws! The bigger the tank and the more heavy-duty the tank and lid the better for the latter purpose!
There are also specially made shelving and cabinets that aquariums can be built into to avoid anything or anyone (such as your cat) falling into or otherwise getting into the aquarium! Many pet stores can direct you to someone in your area who can build this for you if you ask in the aquarium section. Take a look at the link below and click on the "Rooms" section to see a photo of an aquarium built into the cats' boarding area which they call "cat t.v."... as I often do! Hee hee! (Well, it's funny until someone - namely the fish - gets hurt; so we have to do everything we can to avoid that! Looking is o.k. - just don't touch! :)