Both Ryan and I have had many apisto's die from some random disease with symptoms that include red swollen lumps on the skin, waisting away, spinning around on the spot (Chasing there tail), white lumps and white fluffy sores. Theses all fit under the broad descriptions for many different diseases, one of which is Columnaris or Cotton Wool Disease. Other symptoms include whitish-grayish coating on the skin, ulcers, damaged fins, patches on the gills, and erosion of the head tissue.
If it is this then an antibiotic treatment, and disinfecting of the tank is essential.
My personal guess is that apistos are a lot harder to keep healthy than they look and because most are relatively new to the hobby that they haven't yet been domesticated enough to live in standard water.
It could also have a lot to do with diet and the lack of something specific.