Hi @tomsouthall...another dead amano today (just one, at night again).
I think we may be homing in on the culprit! The clue is "at night again". Take a look at the following and what do you see? NOTE: Scroll down the page to "Lifestyle & Habitat".
Planaria - structural features and vital functions of a freshwater parasite, methods of dealing with it
White or milk planaria is a primitive invertebrate creature belonging to the class of ciliary worms. It is found in freshwater bodies of water or aquariums, where it can be seen with the naked eye. White Planaria is a free-living organism that, unlike most flatworms, in its natural habitat poses...
"The lifestyle of the white planaria is mainly nocturnal; during the day, creatures hide among the leaves of aquatic plants".