Aquariums: Sick fish: help!

Hi folks,

I had a mild disaster earlier in the week – my canister filter died. Luckily I have two filters running, and my old Eheim Pickup 200 carried on plugging away. It's far too small for my tank but at least there was still some movement and filtration.

I got a new canister within 24 hours, a Fluval 407. My ammonia and nitrite are at 0. Nitrate is quite high between 40-80 I'd say.

I have one ember who I noticed was behaving a bit differently a few days ago. Now they're swimming at 45°, near the surface, and have a white spot/dot on their right side. Fast breathing. It looks like a scale has popped out more than any sort of growth.

Is he a goner? Should I take him out? I don't have a quarantine tank. Can I treat? All the other fish seem fine. Some Corys acting a little weird – a few times I've seen one scrape it's side against a rock, but that could just be Corys corying.

The rest of the Embers are fine.

Stock – Embers, cardinals, Corys, 2 pearl gourami, otos, one Hillstream, a few snails and shrimp.

Temp is around 23.5°C