You know those good tablets you can stick to the glass? Tetra sell ones called Fun Tips - I wouldn't use them as a regular food, but I have used them to get photos of guppies before, since you can stick the tablet in a good spot to avoid aquarium lights glaring into the camera, and it brings the guppies to the camera focal point rather than trying to follow a swimming guppy with the camera.
Just stick the tablet about 2/3rds of the way down the aquarium glass, low enough that they're not hovering at the surface making it impossible to get a shot, focus your camera on the tablet, then snap away as they gather round to eat. You'll get a lot of dud photos, but some good ones too.
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