Task: Question Answering
Input (English): Context: His head looks huge and his legs ... I ' ve never seen his legs look so weird . I figured another salt water bath was in order , although his stained skin ( from dirty tank water ) was improving since the last time . Also , he 'd been peeling , which is normal . At least , I thought he was peeling . Turns out he 's covered in a layer of brown slime . And his thick , rough skin ... well ... it 's rather soft ... His shell is intact , though , which is the main thing . I was rather annoyed . While I ' ve been adding at least 3 gallons of fresh water to his tank every week , it really has n't been completely changed out since we moved in back in February . 
Question: Why was the animal 's skin stained ?
Output (English): The animal was in dirty water .