Yes, it could be mucus. Loose cobwebby bits like you describe. Also the eye is cloudy. I'm thinking water problem too but have no idea why this would be. Tank has been very stable for about a year. Did an emergency water change and clean last night and it did seem to be grubbier than I would have expected. No sure what has gone wrong. Just going out now to fish supplies shop to buy testing kit and hopefully some magic fish medicine. I'm gutted about the dead fish - it was so beautiful and not a mark on it. Remaining fish is looking a bit distressed this morning. Not interested in food. Not doing its normal pottering about. Just floating there in the middle of the tank breathing.
How do you mean by extra skin? Fish have a mucus layer protecting their skin and if the water is in any way not right then sometimes they produced extra mucus- it can hang off them a bit like cobwebs. Do you think that might be it? Have you tested the water?
We have two goldfish in a 90L tank. DD accidentally acquired them as tiny tiddlers from a fair a year ago. We didn't think they'd survive but they did and have gone from strength to strength getting large and sleek. Just got back this evening a found one lying totally dead at the bottom of the tank. It was fine when they were fed this morning, no sign of sickening for anything. If anything it was the sleeker and fitter looking of the two. Have just given it a look over - can't see any obvious signs of illness - no fungus or weirdness in the fins or scales. It looks perfect. Any ideas about what it could have caused an apparently healthy goldfish to suddenly drop dead?
Plus, now that I look very closely at the surviving goldfish, there is something odd going on around its eyes. Looks like some extra skin around there, with a few tiny bits hanging off. Is this a kind of fungus? What can I do, or is this one doomed as well?