Hi Helene,
I´m not an expert with diseases but I remembered a thread in the IGL forum in the
Paradiesfisch section. Some Macropodus spechti were showing dark spots on fins and
body.
The reason was identified by Charly Roßmann to be organic material like
rotting leaves,- wood,- plants and lot of detritus(don´t know if it´s the right english word = Mulm) that pollutes the water and can cause this spots.
The author of the thread found the source a rotting peace of wood and changed regularly 50%
of the water and the spots were more or less gone.
Here is the link, maybe you can compare the symptoms and it might help.
IGL thread
Volker