Take a large clear plastic container, the type with a screw on lid that carbon/magnesium/etc. come in work great. I drill a couple small holes in one side and stick the suction cups from maxi-jet powerheads in the holes. I drill another hole, big enough to put food through, and small enough so the fish can't get through it. I use acrylic tube to put food through the hole.
Stick the container to the glass, add some food - winner, winner, fish dinner. Put a net over the mouth when it goes in to eat. Works every time for me.