my cats do that as well. they both drink from my fish tanks. none of my fish have had any issues with it. i have not seen any of them get sick from it or anything. they seem to enjoy it. my one cat (Sassie) sits on our bathroom counter and watches my 10 gallon divided tank that has 2 of my males in in, one is my purple and white halfmoon plakat named Purnurple and the other is my red and blue cambodian halfmoon named Missile. They don't mind her. I don't see stress stripes on them or anything to indicate they are stresses do I don't bother to keep her from them .
America (f, HM), Spirit (m, DBLT), Rescue (f, VT), Missile (m, HM), Purnurple (m, HMPK), Dragon (m, VT), platies, mollies, sorority tank of 21 females, AND lots of guppies.