The cat might also like the taste of the water. Some animals are quite the connoisseurs.
My dog prefers to drink "old" water from a big basin standing outside, one cat enjoys licking up the water puddles beneath my flower pots and the other cat prefers fresh tap water.
The chemistry of tap water differs so much from aquarium water that even you could probably taste a difference. Temperature and the material of the container also influence the taste of water.
Experiment with different containers (glass, plastic and metal bowl) and fresh and tepid water. The drinking fountain mentioned by Henders is also a nice try.