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If it was a prolapsed intestine it ripped off the fish, seen it on the sand bedHi, Welcome to Reef2Reef!
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Some Helpful Hints When Posting or Responding to Threads in the Fish Disease Treatment and Diagnosis SectionAquarium Fish Disease Problem Solving Determining the best course of action for a problem with your aquarium can be a difficult task. Even advanced aquarists will often need to seek the help of others for complicated issues. Our intent here is to offer expert advice for your fish disease...www.reef2reef.com
How long has the fish had this?
Is it still feeding?
The two main possibilities are: a prolapsed intestine or a large piece of feces (perhaps from eating something that couldn't be digested).