Help with strange Platy behavior

[quikv6]

I have a platy who is about 7 months old, and acting very strange. He either sits on the bottom gravel, or up top at water line, and really doesn't swim much at all, though he certainly can. I know something is wrong because he isn't interested in food.

But aside from the behavior, there are no visible signs to  point me toward a problem. The platy has good color, fins are upright and not clamped, and I see no signs of fungal/bacterial issues. Visually, he looks great. No flashing either.

Normally, I'd say possibly parasitic, but his community tank has been through 2 precautionary Paracleanse treatments within the last month or two. It has also been through a Maracyn treatment. He was the only fish acting this way in the tank, and there's probably 40-50 other platys. There have been no outside fish added to the tank...all were born in the tank.

**Parameters:  Ammonia - 0, Nitrite - 0, Nitrate 15-25, GH - 200, KH - 100, PH - 7.6-7.8

Any ideas? I'll try to upload a picture...but visually, I am clueless, as he looks great. Thanks in advance.

 

[Kirsten]

Lethargy, unusual behavior, hiding, disinterest in food are sometimes the only signs we get that something's wrong. If possible, I'd recommend quarantining him separately for closer observation and give him 1 tbsp aquarium salt or marine salt for every 3 gallons or so to see if that helps.

[quikv6]

Thanks.  I failed to mention that he's in with some salt,  as he's always been.  I generally use a teaspoon per 5 gallons, though I moved him to a grow-out tank, which is closer to a tablespoon per 5 gallons. 

He can swim fine.....he just doesn't. The lack of interest in food is really most worrisome. Something's wrong...just have no idea what.