Just returned from vacation .Woke up this morning to find my pleco belly up on gravel.He rolled over but definetly looks like a problem.I noticed he didnt eat any of his food last night.He was ok when I left four days ago .This fish is my oldest fish in the house almost 6years .He has always been healthy .Now I have this out of nowhere.He has been thrashing around tank like hes in pain and is now laying upside down breathing rapidly.There is no outside obvious indications of disease or trauma.I have no idea where to even begin with this.Im a pretty experienced fish keeper but clue less as to what Im dealing with.All parameters in tank are normal.6.8 ph ,0 ammonia ,0nitrite,20 ppm nitrate,kh 70ppm,gh 70ppm,temp 82 f.Any help would be greatly appreciated.