we used to have snails in ours-- they were supposed to help with the algae, but, well... I guess our tank pwnd them ;) It could have been there was an egg hitchiking in the last load, or maybe a baby that was SOOOOOOOOOO WEENSY!!!!!!!!!!111!!!! that it was easy to miss.

Better that then a darn mantid shrimp. Those things can break the aquarium glass =:0