Please help me. Fish getting skinny in days.

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I have a couple royal plecs and a motoro in a tank by them selves. In days, the plecos have an extremely sunk in stomach and the ray stopped eating. The plecos are a few years old and have always ate well. I haven't introduced anything into the tank recently but its definitely seems to be an internal parasite.

Please help as the health of my fish are deteriorating fast.

 

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No help at all..