catching fish

[JohnM99]

Buy an orange juice container - one that is an elongated oval, a bit narrow - then buy a cheap mirror and mount it on the bottom, opposite the opening - suspend it in the tank sideways with a bit of food in it, thread a string through the lid to close it - when the fish (particularly an aggressive small fish like a damsel - swim up to the opening and looks in, he sees his own reflection in the mirror, and charges in to scare the "new" fish away.
Voila.
Cost - less than $5.