65 liters is about 17 gallons, way too small for a pleco and silver shark. Plecos produce a lot of waste and get too big for that small of a tank.
You should do water changes every week for good water quality.
Does the red thing on the fin look something like this?: http://www.fishjunkies.com/Diseases/furunculosis.php
Current fish (includes all 3 running tanks): Polypterids, severums (of which I raised from eggs), gouramis, plecos (gold spot ones & a gurupa), loaches (zebras, yoyos, & a kubotai), macculocchi spotted silver dollars, an African butterfly fish, & Ctenopomas (a leopard & an ansorgii)