Long time lurker, first time poster - have learned a lot from you all over the past year!
But on to my reason for posting... I brought home 3 clowns from my local Maidenhead Aquatics on Sunday, and one of them doesn't look good at all

It's not eating, quite skinny, clamped dorsal fin, frayed tail, and just hides a lot. He's now living in my hospital tank on his own, as I have just noticed a slight nitrite spike in my main tank (due to over-zealous filter cleaning, I'm afraid).
Water parameters are Amonia 0, Nitrite 0.3, Nitrate 10-15 (tapwater is 10+ here in Hampshire), temp 25. The hospital tank is 0, 0, 10-15, also 25-26 degrees C.
My question is, any idea what's wrong, and what else can I do for him? He's not flicking, no sign of ich, no apparent fungus etc. All my other loaches in the main tank are fine, including the two that came back with him.
Feeling a bit worried here, hoping it's not some kind of internal parasite

Thanks for any advice,
J