I just don’t see excess mucous on this fish. Why only on the mouth?
Post 1 of the thread at the following link has a video with the angelfish. About 11 seconds into the video the OP zooms in on the upper back of the angelfish and you can see a cream coloured area on one side. That is excess mucous, probably caused by external protozoa.
https://www.fishforums.net/threads/...w-white-spot-on-my-angel.454238/#post-3837091
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The only fish I had with heavy body slime or excess mucous was as a child and the slime hung off the fish’s entire body. When I Google “excess slime in freshwater fish”. I keep coming up with Slime Disease which is usually involved in parasitic disease according to info. Interestingly, the recommended treatment is Kanaplex or Furan-2.
Dr Google is not always the best when it comes to identifying things. I don't recall coming across any diseases called Slime Disease in my fish health books. As for it being caused by parasites, anything that uses another organism and gives nothing in return, is a parasite. This can be intestinal worms, external protozoan infections, Anchorworm, fish lice, fungus or harmful bacteria.
Anything that irritates the skin of the fish will cause the fish to produce excess mucous. Normally it's caused by external protozoan infections, but poor water quality and chemicals in the water will also cause fish to produce more mucous.
External protozoan infections generally attack the dorsal side of the fish first and start off in small areas and slowly spread outwards. The infected areas appear as cream, white or grey patches on the body.
If fish are kept in bad water or exposed to chemicals, they produce excess mucous over their entire body, fins, eyes and face. The fish get cloudy eyes, a white edge to the tail and fins, and a cream, white or grey appearance over their entire body. This is quite noticeable on dark fish like black mollies, who almost look like they have a semi transparent layer of white cream over their body.
As for using Kanaplex and Furan 2 to treat "Slime Disease", you really need to identify the problem before adding chemicals or anti-biotics, and the information on Dr Google should say this. Throwing anti-biotics or any chemicals into a tank with fish that are producing excess mucous caused by poor water quality won't do anything. Adding the wrong medications/ chemicals to treat diseases might fix some secondary infections but won't necessarily fix the main disease.