All of my tanks are open topped (need to be in our climate, and also because 99% of them are second hand). My older cat loves fish food but doesn't go out of her way to drink the fish tank water, but she does drink out of my pond and old bath tub that doubles as a pond. So do the dogs, in fact that is one of the reasons I am for ever topping up the bath tub, thanks to the dogs gulping he water down.
My younger cat (he will be 2 in April) is a total fish tank addict. He does the rounds of my various tanks sampling the water in all of them. He sits on the tanks glass centre support dangles his paws in the tank, lays his tail across the surface of the water and has a good long drink. The endlers in the tank invariable think he is feeding them so they all swarm about it tongue, paws and tail. He is also fish food mad as is one of my dogs.
In the act of moving from one tank to the next one.
The main reasons pets will drink all the water they can out of toilet bowls, fish tanks, ponds, puddles etc is that they just like our fish don't like all the additives that we humans put into our drinking water to make it "safe"
And just to prove that my cat is a bit of a nut job
To get on the timber door he rockets up the front security screen door.
Cats got to love them