It's not supposed to tap, if you're hearing a tap, that's in correct. The whole point of that scene is to feel the fear that being a tiny little fish does when someone huge like that bangs on the tank. Also when you hit a panel of glass it will vibrate and make moe a "DUB DUB" sound as you called it, try hitting a big wood table that isn't very solid, same thing. Because of Darla's size to the fish, a tap wouldn't resonat as well. Also it's a underwater sound which would distort the sound even more. Finally a "tap" sound is produced by the finger nail usually, and a hyper child wouldn't have that much control of their hand, they usually poke things with the finger.
Cheers!
- D
