The fish are my only pets but I have a 2 and 3 year old who both do as that cat is doing. Also they often driving toy cars along the glass sideways. I keep explaining that cars drive on the ground but they do not seem to understand.
but don't see it as a problem
We have Nine Cats (yup that's 9) all will sit on the sofa and watch for hours, but 2 of the females, still think (if we are quick enough, we can catch dinner) but always foiled by the glass!!
It seems to be the Corries and whip-cats which get their goat! they don't chace the Discus?