I read that article & it says “Straight up, if you see little tufts of cottony stuff that resemble white-colored bread mold, it isn’t columnaris.” But a lot of the other identifying symptoms are there. I went to a fish store (much better then petsmart or petco, etc) yesterday before I saw your response & they recommended Furan. I immediately started treatment but having not read the article yet, I medicated the whole tank! Ugh. I can’t do anything right for these fish. Im pretty sure it’s too late for the poor eel but I feel awful just letting it die without trying help it. If it was you, would you stop medicating? Or would you get the KanaPlex & continue to try? There’s only the eel & 3 orange & black Molly’s left. I’ve spent a lot of time & a lot of money on this tank for it to be completely depleted in less than 2 weeks!! Also, we woke up to a bunch of foam on the water surface so I “closed” something on the filter. I’m not exactly sure what it does but it seems to have slowed the water flow. It’s not as bad as it was earlier, but still foamy. Is this from the medication?If I had to diagnose anything by the photo alone I'd say Columnaris. I lost all my fish a few years ago to Columnaris and I had to start again. I had a break from the hobby because of it, it's devastating. Again, please don't take my diagnosis as gospel, but the speed you're losing fish and the cotton like growths are similar to the symptoms of my fish. Columnaris can wipe out a tank in anything from 24 hours to a few weeks.
Have a look here... Identify and Treat Columnaris (Saddle Back Disease)