One of my three Leporacanthicus which i keep in a 100-43-56cm tank along with 3 clown loach died on Friday



I found him dead at 11:00pm.I tested water parametres and they were ok(pH 7,6, NH3 0, NO2 0, NO3 20)I recall 12 hours earlier he was breathing quite fast and he had a very rounded belly(he/she had a very rounded belly almost 6 months now but he was acting and eating normaly and he always was the first to reach the food)Also he and one of the other Leporacanthicus were hiding behind the external filter inlets at the mid to upper water layer but they hide there about 3 months now.He was eating and acting normaly even the day before he died.
I would like to know what killed him because i dont want to loose the others from the same illness.
On Saturday his friend with which he was hiding behind the filter inlets started breathing a little bit faster than normal and he moved and hided some cm below the surface,so i increased the aeration of the tank and after a while he moved back where he was hiding.Today he did that again and i further increased the aeration.He does not has a rounded belly and he hasnt eaten 3 days now.
Is he ill or not?If yes how can i treat him?
Thank you in advance