Aiptasia stings will cause a wound that gets bacterial infection and begins to erode flesh.
I'd NEVER alow Aiptasia to remain in my tanks.
As for parameters that cause GBD, like bacterial problems, it's not something that can be measured with any hobbyist test kits.
From experience, I, like many others, have learned that you need to keep the water cleaner than any reef tank has need of, frequently removing all uneaten food and detritus and doing large frequent water changes. Oversized skimming is also a big help.
You won't have the problems from insufficient husbandry in the short term, but conditions deteriorate over time so that when they become afflicted, you don't understand why because you haven't changed your protocols and feel you haven't done anything to cause the problem.