In the past and before there was "cruelty free cycling," lionfish were used to cycle tanks along with damsels and they survived just fine. Not to say I believe in this practice, but they are relatively hardy animals that can readily survive in over 100ppm nitrate. The only major concern in sw
as that can do extended damage to their gills. In saying that, I'd feel comfortable keeping any lionfish under 40ppm so I don't think water parameters are the cause. As is often the case, most diseases are introduced and that is likely the cause here. Most diseases will go unnoticed until its too late (observed by naked eye) and most can kill in under 24hrs. if not treated prior to.