Does tapping on a fishtank really hurt/kill the fish inside?
No, but it’s certainly louder to the fish than it is to you! I doubt there is any physical evidence of it killing fish…
It will stress them, and depending on the species and other stress factors it could be the straw that broke the came’s back before they go belly up. Shining a flashlight across a dark tank for 0.5 seconds will freak some fish out enough to launch themselves clean out of the tank (as I found out yesterday). For many fish the #1 cause of death is stress, and tapping or banging on their tank is a really good way to cause it.
Some can get used to it though, just like we can get used to living right beside a train tack or having megaphones blasting off around us. It’s not a good idea to do it though, especially to other people’s fish.
I tap on the tank with the food container before I feed them. They all come swimming up to the surface and wait for the food. Hasnt hurt them yet. They *do* get upset, however, if I tap and then *dont* feed them.
Just so everyone knows the difference - tapping to say “dinnertime!” is light tapping. When I had fish, it was a light flick a fingernail on the frame (side benefit - keeps fingerprints off the glass that way!) sort of tap. Close your eyes and tap on your eyelids - don’t hit the tank any harder than you’d want to hit your eye.
Banging on the glass with the same force as you’d knock on someone’s door is not light tapping.
My parents turn on the lights to the aquarium before feeding the fishies. As you may expect, learning “lights == food” came very quickly. And they got upset if you turned on the lights and didn’t feed them. 
How does one determine that fish are “upset?” Do they cross their arms and tap their toes? Do they squint their eyes and purse their lips? What? Help me fight my ignorance.
You can best tell in two ways:
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They become agitated, swim fast and erratically, jump out of the water, hit the sides or the gravel
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They stop swimming altogether and their gills stop moving.
I tap on the tank with the food container before I feed them. They all come swimming up to the surface and wait for the food. Hasn`t hurt them yet.
[They do get upset, however, if I tap and then don`t feed them.] 
Better?
And yes the tapping is very soft, (do not – abuse, bash, batter, beat, beat up, bob, bruise, buffet, clap, clout, cuff, damage, deck, drub, fell, flatten, floor, hit, hurt, kayo, KO, level, maltreat, manhandle, maul, mistreat, pound, punch, push over, rap, rough up, roughhouse, slap, smack, thrash, thump, thwack, total, wallop, whack, or wound the fishies.)
While I’m not sure about the tapping, I can tell you, from personal experience, that if you place a fishbowl on top of a large speaker and play Metallica at volume 11 for a couple months… The Fish’s balance “bubble” inside can burst, causing it to swim crookedly, or not at all. This was a poor fish my friends had in college. When they gave it to me to “babysit” while they left on break, I put the poor thing out of its misery.
(I played John Denver instead.)
I’m just kidding about John Denver.
A stressed fish on its way out the door will swim erratically too. I never heard of the Balance Bubble though.
I’d say there’s nothing inherently wrong with the noise level of moderate tapping. It’s a signal that can be good or bad: either a startling one that makes them freak out and stampede all over the tank (probably hurting themselves), or a nice one that makes them freak out and frantically look all over the tank for the food to come.
It is right however that neither of these signals should be given by curious people just to see a few seconds of ‘action’. If you don’t know what will happen, don’t tap. But if you always have to open a rattling cover to feed them it’ll be a harmless “food” signal anyway. 
The worst thing that will happen in this case is their disappointed looks when there’s no food. It may build their character, but can you live with that?
So, what about the noise from the filter? I just explored a fishtank with a stethoscope. It should give an idea about what one would hear in the water, at least the relative noise levels. Whoa. The loudness of this constant rumbling and humming is comparable to that of tapping against the tank, depending on where you tap/listen. I wouldn’t expect them to close their eyes at night…
Are there any observations that fishes living near internal or hang-on-the-back type filters are more nervous?
As a kid, I noticed that tapping on the glass didn’t seem to even get the fish’s attention, unless the fish was directly next to the glass - in which case the fish was probably more freaked out by the giant finger coming out of nowhere than the noise.
Thanks for the replies all.
Here’s a new question. If you do a search for “tapping tank” (w/o quotes of course), the above post does not show up, even though those words are clearly in the title. I know this because I tried it before I posted and I have tried it just now. How did you get that link gotpasswords?
Why do people find it necessary to get the fishes’ attention? In my tank, the fish are always looking out for food. If one of them finds some floating on the water, they all seem to know about it instantly.
Does anyone really have a tank full of stupid fish who wouldn’t notice food floating past them without a little heads-up?
You`d like them to eat most of it before the filter gets ahold of it, or it falls into the gravel.
i do it so all the fish get equal food
Some fish are quicker, eat msot of the food, and the other ones dont get enough. Tapping gives them all the heads up.
I’m pretty sure he’s referring to the swim bladder, the pocket of air that allows a fish to regulate depth and balance of swimming.
My fish are cleverer than your fish! All ours go to the corner of the tank where the removable panel is everytime we go near it, especially in the evening (feeding time). They must have strong characters as the TV’s right by the tank so every time we go to turn it on they’re there!
A little disconcerting really, realising that you’re constantly being watched by 48 pairs of eyes…!