Angel Fish Ulcer?

[Jakkariah]

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

 

[jlk]

Hi and welcome!!! 60l is only @15g- I dont keep angels but this is an issue. What are your exact readings for ammonia, nitrite, nitrate & ph? What are you using to test (liquid or strips)? How exactly are the filters being 'cleaned'? How many fish are in the tank? Your fish has a serious infection but I dont know what meds you have access to. Your water parameters are very important here because without healthy water, meds are ineffective. Some more info will help us to help you better!