Well, in my 18 years of seahorse keeping, I've never found a way to determine when my water quality is getting to the point it's causing seahorse problems.
THERE ARE NO TEST KITS available to the hobbyist to tell us when the water quality is degrading to the point bacterial problems are going to happen. (things like dissolved organics) You can test for all elements you have kits for, and the basic three ammonia tests and it still will NOT give you that information. That is why we have to be pro-active, not re-active in our husbandry practices. Pouch emphysema is often the precursor to letting you know you may be in for more major problems coming up.
As for having others with no pouch emphysema, that is entirely probable.
Seahorses, like people, have varying levels of immunity, with some seeming to never get sick, and others seeming to be continually problematic. Others will fall somewhere in between those points but with seahorses most will be closer to the problem end than the good end.