The typical parasite for polypteridae is Macrogyrodactylus polypteri. It is a type of freshwater leech. It lives from surface cells of the fish. Too many parasites will kill the fish. The leeches are 3 to 5 mm long, very thin, can hardly be seen with the naked eye.
Treatment (source Jurassic Fishes - Haruto Kodera): You should prepare a quarantine tank and add 250 ppm of formalin solution (formalin solution on the market diluted to 1/4000th strength) and then bathe the fish in the medicated water for 30 minutes. If the parasites are not exterminated the first time, repeat the same process until they are. You should be particularly careful in the case of a mixed tank. In no time at all the parasites on a single infested fish can spread to other specimens, so it is a good idea to add some fish medicine to the regular tank too. Based on my own practical testing, half of the prescribed concentration of a product called Green F Gold is most effective.
I have tried the Green F Gold in my tank with the half concentration, as I had only polypterus senegalus, and they were all affected. Aditionally I turned off the filter, but added an oxygen pump, as I was told, that the medicine would use up lots of oxygene. After 3 days, I turned on the filter again and the tank cleared. The parasites did not survive the procedure, the senegalus had no problem.
BG
Thomas