I was actually at Wal Mart (AKA Fishie Hell) and there was a teenage couple tapping on the galss and thought it was funny I told them that it really stressed the fish out. They looked at me like I was crazy and walked away. Oh well F)&* em they should know better those fish are in bad enough shape already.
I hate it to! I will usually tell them not to once, then if they do it again and I happen to be around I will walk up and give em about 3 or 4 rapid pokes on the back of the head. I ususally get the ouch! Hey what did you do that for? Then I say why did you tap on my aquarium after I told you not to?
Well i agree with all of you so i use a drop of something called koi kalm in the water keeps the stress level right down but i only do this when ickle kiddies are over (7 year old girl that loves to tap told her not to but bet she still dose it when i'm not watching)
Occasionally my four year old will put her hands on the glass of one of the tanks, but not very darn often. I had a visitor once who thought it was clever to tap on the glass.... for about half a second.
Nothing drives me crazy like a child banging on the fish tank at the LFS, and the proud Dad saying how much the kid loves animals and will grow up to be a zookeeper etc... Really gets my goat! Especially when they decide to pick on the poor Discus 'iv tha fishy ded dada' bang bang bang 'wake up mister fishy' bang bang bang... awwwwww lets get him one honey