Ermm, all three of my cats do this. They dunk their paws in the waterfall area too. The bettas act all tough at them, the gouramis run away, and the livebearers run up thinking they're gonna get fed... but kitties' paws can't get close enough to cause trouble no matter what.RaeJeanne said:and (you may gasp) our cat drinks out of it. She can't get to the fish because the only part exposed is the part where the filter waterfalls it back into the tank and she drinks from the waterfall.
Anyway, I would worry about water that's not changed out regularly is going to become saturated with heavy metals, which don't evaporate with the water. Plus, I get the poo and whatnot from gravel vacuuming the water. AND finally, gravel vac and water changes help remove the ICH spores so that the problem doesn't continue once the heat comes down to normal.