Blackness

Blackness is a highly infectious and deadly desease amongst Syreni that rots away fins and rots away teeth, eyes and gills. Infected Syreni are cast out of the City of Aogua and shipped to one of many Blacksites all around the city. There, the infected are left alone to die. They used to be the main population of the Blacksites but as "going black" (a term for losing your citizenship) started to become more and more of a problem and convicts were exiled to these Blacksites, the sick were cast out of their homes in the everpresent coral reefs, and pushed back to the very end of the Blacksite planes.

In Blacksite ⧣2369 for example, the infected had to retreat into the Blacksite #2369 Cavesystem which they had used mainly as a storage facility in the beginning and now serve as their only shelter. They aren't perfect for living because Syreni don't have a well developed way of manipulating stone, and there is no automatic supply of oxygen like in coral reefs, and the caves are highle susceptible of Submarine Earthquakes which can lead to whole sections of the cavesystem just crushing their residence beneath them.

Blackness is highly contagious and there is no Syreni who is immune. Even semi-sentient ocean inhabitants are capable of being infected. The only living organism not being at risk, seems to be Andre Dolan, a human.  The sickness is transmitted through touch or the exchange of fluids. That is why typically no Syreni risks going to these colonies. 

There are some though, mostly Syreni Architeuthis or Syreni Hamiltoni who go there as a communal service to look after the sick as best as they can while maintaining their distance of course. For this reason, there are special suits made but their fabric is thin and quick to malfunction as the technology hasn't been developed far enough yet. Because no one really cares about the infected, at least not enough to pour money into helping them.