That looks like a body ulcer ... the white spot is indented with a chunk of skin missing.
1. maintain very clean water - ulcers often happens with high organic debris content in the water. In spite of your good nitrogenous waste level, an extra water change or 2 won't hurt.
2. Clean ulcer with either 3% H2O2 or betadine ointment. <One application with a q-tip. You will need to lift the fish slightly out of water. Put a clean (wet) towel sling under the fish & gently lift up so the ulcer is out of the water, clean quickly.>
3. Check for parasites- this is ofter the cause of an ulcer - look closely at the wound .. anything hanging out? <It almost look like there is a forked worm - anchor worm - in the wound.> Remove anchor worm with tweezers if there. If you find nothing, you might consider treating empirically with a salt dip or with Prazi. If you find something, the treatment depends on what you find.
4. Watch the wound for bacterial infection - redness, lethargy, etc. A low level of salt (0.1%) may be used to decrease infection till the ulcer heals.
80 gal FW with 30 gal DIY wet/dry/sump.
9 fancy golds, 1 hillstream loaches, 1 rubber-lip pleco (C. thomasi), 3 SAEs, small school of white cloud minnows, planted.