Tank info- 55g display, 10g sump. Established for 2 months, cycled with live rock and sand. Inhabitants are the blue dot puffer and 2 clown fish.
Salinity 1.025
PH 8.2
Ammonia 0
Nitrites 0
Nitrates 10
Water changes 50% a week divided in 3 changes.
I’ve had the puffer 2 weeks, he’s been flashing occasionally since i brought him home. Fish store said they just do that sometimes. He developed what looks like a scrape on one side that i attributed to his flashing. Now he’s gotten some dark areas on each side and near his tail. Fish store says that means he’s happy and I’m overreacting

. He seems to eat great and act normal ( I’ve never had fish, but i think he’s normal). The dark areas look like his skin as opposed to something ON his skin. I’ve never seen him poop, so can’t say how that looks, but never seen anything odd in the water. When he flashes it looks more like he’s aiming for his face then his gils, but not sure. Could a gill parasite cause the skin changes? Or two separate issues?
I have photos, but it’s saying file too large and i don’t know how to make them smaller…