Sick Severum with wounds

Hello everyone,

This is my Severum, usually stays in a 4ft tank (200L).

I have no substrate or decorations, the only thing in the tank is a submersible pump and heater.

I had noticed some white spots on him (first two photos), he seemed to sit in the corner.

I had gone to my local aquarium shop, (now closed due to lockdown). And he recommended methylene blue.

I had used this on my main tank, after 3-4 days as specified I did a water change.

Before the water change I had found these white worms? Sitting on the glass (picture 3), after the water change they are now gone.

I had noticed a bump day 2 of the methylene treatment, by this day he is now no longer eating and is still sitting in the corner (next to my heater).

I noticed that my Severum was very inactive after water change (25%). I had moved him to a temporary/hospital tank.

The last photos are the current state of him.

I am unsure what to do at this stage, it looks like a deep wound but I'm unsure what it could have been from? Heater?Filter?Glass?

I am getting worried as his health seems to be deteriorating.

Quick update, the wound has gotten bigger and his eye is starting to get cloudy and bulge.

He doesn't look to be improving.

I may have to euthanize him, he is very lifeless. Isn't eating either.

Not looking good, Fish need a base in their tank and rocks and plants they can identify with. Think about what their natural environment is, that is what their tank should look like. Not a glass box.