ray with worms

[john C]

have two 5'' motoro's in a tank one had for approx 3 months other had for about 3 to for weeks newest one acquired and was wormed however in a tank with several others so possible not wormed ? older never wormed. girlfriend has been complaining about olders behavior.(not swimming as much and hiding in sand all the time ) i found a white what looked like a piece of thread about 1.5 inches long sticking out from sand and next to older ray. pulled it out of tank and it was moving internal worms right? have some metronidazole by seachem treatment for several protozoan and anaerobic bacterial diseases. should i use in food if so how much. hope you can help thanks john