It's not a good user experience when you feel you've been chastised by a computer program. All I did was click on the fishy for heck's sake!

It's a bad idea to make users feel stupid. There are a bunch of things these guys might have been able to do to improve the interaction:

• Let me click on a fishy, then change the appearance to indicate I'm supposed to drag it into the tank.
• When I click on a fishy, just put it in the tank - I can always remove it! (While this is more usable, some would argue it is less engaging - I get a better feeling of creativity if I place the fishy myself.)
• Make the fishies look more draggable in the first place (affordance).

But at the very least they could re-word this message so I don't feel like I'm in grade 3 being punished by Mrs Brown.