My cats drink from the chagewater, as well as directly from uncovered tanks. It's never caused any problems for them, and this includes one that lived to be 18 years old. There really isn't anything in the water that's going to cause them a health issue normally. If you are medicating the tank, then I would prevent the cat from drinking that water--things like copper won't be good for any animal (it's not really good for the fish, either, and should only be used as a last resort).