HITH can be the result of numerous causes, some underlying issues include thiaminase poisoning which eventually block tissue reformation and leave fish susceptible to more permanent-esk damage over time. HITH can also be a result of increased stress from poor water quality as
duanes
said, and result in heightened responses towards it. this period of weakness gives members of the Spironucleus group an opportunity to multiply and slowly chip away at both tissue and the mechanisms that repair it. SP v/spp is found in most fish through their intestines, but are harmless until given the chance when immune systems are weakened.
considering that the water parameters are stable now I would check for any noticeable issues or telltale signs of parasites or other unwanted parasites along with other issues that may cause unnecessary stress towards fish; ie external effects like sound, excessive vibration, or even movement that may push the fish into a panicked state.

considering that the water parameters are stable now I would check for any noticeable issues or telltale signs of parasites or other unwanted parasites along with other issues that may cause unnecessary stress towards fish; ie external effects like sound, excessive vibration, or even movement that may push the fish into a panicked state.