Hole in the head? Lionhead Goldfish

[CodKing]

My poor mate Tippy looks like he has hole in the head and I dont know why :? Water quality is good and tested often, tank is 50L(ish) and has a few plants and 1 other lionhead. 1x15W 6500K light is on for 6 hours a day and water temp is 21- 22C. Fert is Flourish Comprehensive @ 2ml every couple of days. Filter is an aquaone aquis 500. Water changes are 10L every week/week and a half. He has been through a couple of ammonia spikes from earthquake issues and a filter screwup from me.

I'd be super gutted to lose this guy, I've got a spare tank I can put him in if need be, I'd even totally change the current setup just to make him happy.

[fishplants]

Feeding regime? HITH is cause by nutritional deficiencies resulting from poor feeding/intestinal parasites.

Also, damage and subsequent fungus can leave a crater that looks identical to HITH, but isn't. Although, just having a closer look at the second picture, it looks like another patch beginning to the left of the crater?

[CodKing]

Thanks for the insight. Yes it looks like another one starting to open up. The first has been very slow to get to this size, I thought it was just a bit of an irritation, I only noticed it was an actual hole when I put a torch on him. The tube I have is a Sylvanian Gro-Lux 6500K which makes them glow and it's not so noticeable.

Feeding: Wardley Advanced Nutrition 'Perfect Protein' two or three times daily. Was also feeding TetraFin sinking mini sticks (for ornamental goldfish) but had slowed it right up as they seemed to be missing a lot of them and the water quality was suffering. Have since restarted adding only a few sticks with every feed.

I've ordered Seachem Gold Buffer and Gold Trace and added a 5cm2 chunk of dry base rock I had floating around. Will test calcium again tonight, on sunday I was getting a result on the first drop of test solution.