Scale worms
Transversotrema sp.
For example, described in the book "Gesundheit für Zierfische" by Mehlhorn / Springer Verlag, and not so rare recently - or quite common.
For diagnosis, you have to observe the fish closely to see if they have some scales that are whitish/cloudy. They are particularly noticeable on black drawings, otherwise they are best seen against the light. The worm pushes itself under individual scales like a tick and sucks there.
Here is an extract from the book mentioned above:
Some freshwater and saltwater species, common in neons
Where found: Under the scales
Biology and characteristics: Sucking worm (digene fluke), which primarily infests internal organs as an endoparasite. However, this species lives as an ectoparasite and feeds on the skin and blood of the host...
Hermaphrodite, 3-5mm in size...special body structure (unlike other digene sucking worms) abdominal sucking disc.
Life cycle is linked to aquatic snails which must be present in the tank.
Larvae live in the snails, emerge from there and migrate back under the scales of the fish as so-called fork-tailed cercariae, where sexually mature flukes are formed.
Transmission: adult flukes -> physical contact, new infestation only after previous reproduction in snails.
Symptoms: heavy infestation with flukes sitting under the scales leads to inflammation and secondary infections; mass occurrence leads to anaemia and death, dropsy or FTB.
Diagnosis: Search the surface of the fish's body with a magnifying glass
Prevention: Avoid snails in fish aquariums, examine new arrivals
Treatment: Praziquantel in a medical bath: 20 mg/l for 90 min. 10 mg/l for up to 48 hours at 22°C