Taxon on Jan 25, 2010January 25th, 2010, 3:40 am EST
The photo you posted from another forum is of a damselfly nymph. If the fish in your aquarium are as small or smaller than the nymph, it will likely eat them. On the other hand, if the fish in you aquarium are significantly larger that the damselfly nymph, one of them may eat it. If you don't want the damselfly nymph in your aquarium, either attempt removing it with an aquarium net, or just wait for it to reach nymphal maturity and emerge.
Taxon on Jan 25, 2010January 25th, 2010, 5:38 am EST
That doesn't sound to me like anything a damselfly nymph would do. Perhaps an aquatic beetle or waterbug. Is it possible for you to take a macro photo of the culprit, and share it with us? In any event, your plan seems sound.