There will be aways a reason for melting, of course.
Changes in chemistry, temperature, salinity, etc... could lead to a bacterial/ fungal infections. That's why they melt. It's an infection, or more.
Many people keep changing the polyps around the system trying to figure out what spot they would like better. Until they get an infection because of stress and then they know for sure that "no spot in the tank would be good for that species in particular". Leaving the polyps alone in a chosen spot is the best bet for it's adaptation.
Adaptation and open wounds from fragging, specially with those unacceptable tiny 1 or 2 polyp deals are way weaker and susceptible to infections than small 10-15 polyp frags (also called mini-colonies). That's why frags should be at least 10 polyps on the plug, if so.
Transportation and stress will be a great contributor to infections with shocks. Long shipping periods won't be good! Chopping wild colonies into 1-3 polyp deals to be fast sold are great for infections!!
Predation also could lead to an infection. Even long dips would damage the polyps and make them weak!!!
Your system's qualities will be a good environment for some individual species, but not for others, and so those would have a harder adaptation/resistance to infections because their immune system could be more easily affected that way.
I believe that what matters most is where the polyp came from (wild), not so much what species it is named under. I'm talking about species names, not cartoon names!!!
The original natural environment where the polyps were found tells us lots about it's natural abilities to fight against diseases and to have it's normal reproduction/growth rates.
Too bad it's so hard to know more about their collecting site.
Main thing then is to keep an stable system and avoid irritation on the colonies. Remember, on the wild, organisms like hermits don't stay so long on the colonies. Again we have people buying 30 or 40 hermits to bother small tiny zoa colonies in a 20 or 30 gallon system for 24 hours a day!! No common sense. Watch for predators and dip correctly every new colony in order to keep pathogens and other undesirable organisms from entering the system.
We still don't know much about some of the worse melting pathogens.
And... good luck!
Grandis.