Well...whatever the cause...the reason for him being off colour is that there is .75 ppm ammonia in his tank, presuming you have an accurate test kit.
Quit hoping and do a large water change, to reduce ammonia to 0...100% or whatever volume is necessary. Do it quickly enough and he'll be fine.
btw .75 ppm is not 'the slightest bit' and he's not sick for some odd no-reason...he's sick because his water is toxic.