There are three main types of parasites and all three can cause flashing (but flashing does not always mean that there are parasites). The three types are:
Protozoa/metazoa (internal and external)
Cestodes/trematodes (e.g. flukes/tapeworms)
Nematodes (e.g. roundworms)
The treatment for each of these is different:
Protozoa/metazoa (something like metronidazole 5mg/L, dose once each day for 3-5 days w 25% w/c before each dose)
Cestodes/trematodes (something like praziquantel 2mg/L for 24 hours, repeat in 10 days)
Nematodes (something like levamisole 2mg/L for 24 hours)
If you want, I can tell you how to collect a skin, gill or fin biopsy and you can send it to me to look at under the microscope to attempt to definitively identify if it is any one of those parasites.