could have gotton it stuck in the filter, or it's buddy could have eaten it. newts have absolutly horrible eyesight and if he saw it moving he could have lunged at it thinking it was food. i would take the filter out and put some aquatic plants in. newts arn't messy and they certainly don't need a filter and water changes that often. i have two heavily planted newt tanks and i haven't changed their water in like 8 months and it's sparkling. little guppies keep it clean too. good luck!