Could you try to post some pictures that are a bit more clear? I'm having trouble seeing it, but I'd guess it may be bacterial and it could be Columnaris.
Quick response is very important in treating Columnaris successfully. I would take the temperature of the tank down to 76 slowly, and add salt, bringing the salinity slowly up to 2% and even 3% is better if the fish don't seem to be having any difficulties with 2%.
If it's fungus the salt in the water won't do any harm, and if it's Columnaris the salinity can have a dramatic effect in preventing the spread of the disease and the mortality of any infected fish.
This is a first action based on your description, but when we have better pictures then we can know for sure if it's Columnaris or fungal (probably not) and treat accordingly.
A few questions: Have you had any new additions to the tank? Has there been any unusual stress, such as fluctuating temperatures or spikes in ammonia or nitrite?
If it's Columnaris I don't think Melafix will be sufficient to treat it. Please try to get some more pictures posted asap.
http://www.aquariacentral.com/forums/showthread.php?t=134928
The above link is an exceptional thread discussing Columnaris, posted by one of our fish Gurus, Flaringshutter. Please read this and also try to post more pics when you can, in the meantime bring your water temperature down and try to do the salt.
Table salt is fine, you don't need to spend money on Aquarium salt. The iodine and anti-caking ingredients are in trace amounts and will be no problem.
