One of my sister's students appeared in her lab on Friday with a pic n mix bag of fish - get this - so my sister could put them in the lab tank as pupil about to go on holiday and otherwise they would be flushed

We have been trying to sort them out (I am not there, my sister is not there to take photos until Monday, she has been describing them to me) - all have ID and plans except one she describes as looking 'something like a black and grey angel fish but not an angel fish' ... I am well stuck. It is apparently not a black widow tetra. Mid to upper water preferred. No barbels. Round rather than torpedo shaped body.
Any ideas please so I can refer her to photos online until she can get back to school to photograph it??