Two days ago I noticed a white "pimple" on my largest severum. It was leaking a little bit of white thready stuff. This appeared out of nowhere as he has been fine before this. Yesterday when I came home from work it had popped and left a crater. He didn't eat when I fed and his fins are now clamped. He was also shaking his head a little so it's clearly bothering him. I've posted some pics below so you can see when it first started and then from yesterday after it popped. Is this Hex? I've never had to deal with that before and I always thought it started out as pits or craters. This was like a big zit. Should I order some Metro? I also have Epsom salts so planning to soak his pellets in it. He's in a 135 gallon. Parameters are 0, 0, 20. Ph is 7.6. Kh and Gh are 5dgh.

