The fish is covered in excess mucous that is produced by the fish because something in the water is irritating it. The stringy clear stuff coming off the fins is excess mucous too.
There's probably a water quality issue or you poisoned them with something. This probably caused the death of the other fish.
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Test the water for ammonia, nitrite, nitrate and pH. Post the results in numbers here.
Wipe the inside of the glass down with a clean fish sponge. This removes the biofilm on the glass and the biofilm will contain lots of harmful bacteria, fungus, protozoans and various other microscopic life forms.
Do a 75% water change and gravel clean the substrate every day for a week. The water changes and gravel cleaning will reduce the number of disease organisms, poisons or chemicals in the water and provide a cleaner environment for the fish to recover in.
Make sure any new water is free of chlorine/ chloramine before it is added to the tank.
Clean the filter if it hasn't been done in the last 2 weeks. However, if the filter is less than 6 weeks old, do not clean it. Wash the filter materials/ media in a bucket of tank water and re-use the media. Tip the bucket of dirty water on the garden/ lawn. Cleaning the filter means less gunk and cleaner water with fewer pathogens so any medication (if needed) will work more effectively on the fish.
Increase surface turbulence/ aeration to maximise the dissolved oxygen in the water.
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Make sure you don't have anything on your hands when working in the aquarium. This includes things like soap or hand sanitiser residue, perfume, grease, oil, etc.
Make sure any buckets and hoses used for the fish are only used for the fish and haven't been used for anything else. If you don't have specific buckets for the fish tank, get some new ones and use a permanent marker to write "FISH ONLY" on them and use them for the fish.
