Large lion fish that has for the past 3 weeks swims to the surface and appears to gulp air and then retreats and swims for a while before doing same action. About 5 years old and very healthy eater, skin and fins, eyes all look very healthy. Small number of fish in 400 gallon tank with good filtration. I'm thinking flukes but can they be treated in a fresh water bath if gills are infested with them?