I came home from a long weekend away to discover that my dwarf gourami is exhibiting dark gray discoloration on the bottom front part of its body (belly area and fin). The color is quite dark - almost black - but otherwise the fish doesn't appear to be showing any other symptoms. The affected area doesn't seem to have an unusual texture (no apparent cottony growths, no swelling, etc.) I thought it might be some sort of fungus or velvet, but after doing some research on the symptoms of those conditions that doesn't appear to match up with what I'm seeing. Any thoughts on what might be affecting this fish, whether it's serious, and what treatment if any I should be undertaking? By way of context, I have a ten gallon tank with five glowlight tetras and a single dwarf gourami, plus some live plants. The tank has been up and running fine for about 4 months. I change 1-2 gallons of water weekly and at last test (yesterday) the ammonia level was fine. While away last weekend the heater stayed on but I left the light off to prevent algae growth (although there is still a fair amount of sunlight in the room. Thanks in advance for any suggestions!