ANSWERS: 2
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I do not know what 'stuff' means in the way you wrote your question. Stuff can be anything - yellow is color -granted, but there is a wide variety of yellow from very pale - near white 'puss' yellow to bright lemon yellow. In order for you to understand the problem I'm giving you this link: http://animal-world.com/encyclo/fresh/information/Diseases.htm Scroll down, it lists all common fish illnesses. This place even gives you an idea of what you need to treat the fish. Mind just because they give a color (like white or gray) that doesn't mean that that is not what the fish has. Often white and light gray 'stuff' can looked slightly yellow. Since I can not see the fish myself I can not say if this is a very pale yellow (white or gray) or not. If you are still in doubt, take a photograph of the fish and take it to an aquarium shop or pet store - if the picture is clear enough the staff there will know what it is for certain.
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I found a picture on the internet that looks similar.
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