G'day everyone

I have a clown loach that is constantly flashing (for the past 3 weeks or so). I have two others that are not, and neither are the other fish in the tank.

Been looking out for whitespot - but there isn't any white spots visible.

Treated with Prazi, thinking it was gill flukes, but no effect.

And the strange thing is that it only flashes on one particular ornament (in fact a PVC pipe which is the home of one of the clown loaches).

I perform regular water changes (at least once a week). Ammonia = 0, NO2 = 0, NO3 = 5-20

Any idea what it might be and what I could do?

Cheers
Tim