Our betta recently developed this grey patch on his head. It's not white and feathery like a fungus, and it's basically flat with the rest of his body, not recessed like a hole. It's like a slate grey patch with no scales. His water was quite high in nitrates because of overfeeding so I've changed 20% per day for the last 4 days, and cut back to feeding 1x per day with less food. I used betta fix for the last two days. Also vacuumed the gravel each of the last two days. No improvement or worsening so far.
He also has significant swelling of the abdomen, which appears to be constipation, as his scales aren't 'pineconing'. I was thinking this was a separate problem, also derived from him eating the flakes meant for the other fish, which caused the overfeeding.
Looking at the zoomed-in pictures, I also noticed he has developed fin rot. Could the fin-rot be the same thing on his head?
He still eats fine, although he has gotten more aggressive with the other fish he lives with.
I'm at a loss for what treatment to pursue, because I don't really know what this is, hole in the head seemed the closest from the pictures I've seen on line. I included two pictures for comparison, it's just a smooth grey spot as in the photos. Can anyone help diagnose this? Should I just give him a wide range anti-biotic for the fin rot and focus on keeping the water clean? Thank you.