Have a couple of angel fish in a 60litre tank (bit small but upsizing soon) and i had to go away for a week and a few days but did a water change before I left. My mum fed the fish and cleaned the filter out once. When I got back I saw that one of the angels was a bit bloated and then it turned around to reveal a rather large ulcer on its body underneath its right Pectoral fin. I've attached pictures, not sure if they've shown up tho. I was just wondering if anyone had any ideas as to what it is/ how to treat it? The fish itself is acting normally, eating and swimming around just fine. It doesn't seem to be affected by the ulcer at all but I'm a bit worried about it. Any help is appreciated.
Jack
Jack