I've recently posted about this same exact problem, and everyone instantly began asking about water parameters, etc., which is the normal first response. Mine were all within normal ranges....and no further solutions were offered. Then I started doing some intense internet research. I found that Dwarf Gouramis in particular have become markedly less hardy through generations of over-breeding and are particularly vulnerable to a disease called Dwarf Gourami Iridovirus. Iridovirus is a disease that develops and effects specific varieties of fish. This is exactly what looks like your situation is, as it appears to be exactly the same issue I had in my tank. There are some good articles on the internet about it, and it appears that a large portion of the imported fish batches (mostly from fish farms in Asia) are effected by it. It can be a long, drawn out disease, and by all the research I've done, it is highly contagious amongst Dwarf Gouramis in the same tank, incurable and fatal. Unfortunately, it is a slowly developing disease, and may not show up for weeks or months after purchase...or could show up in days. Just depends on how stressed the fish is. The less stress, the longer it will last. Although I LOVE the look of Dwarf Gouramis, and the articles recommend only buying Dwarf Gouramis bred from certain areas (or NOT bred in certain areas), I've just quit buying them all together. I do know that most of the major large-scale pet stores (Petco, Petsmart, etc) buy their Dwarf Gourami stock from these questionable markets.
