Don't read much into these. Just trying to help you if I could.
TW: No gill flukes,
TBTB: How do you know? So far parasites or infection or bad water (despite the tests) are all I can think of.
TW: I have an api master test kit in the mail,
TBTB: Ok
TW: I was using the three separate to test parameters before.
TBTB: three separate what?
TW: All other fish are super healthy and even the cat has been till now.
TBTB: Is this based on good appetite, looks, and normal behaviour?
TW: Don't know if it's too much competition for food, seems like he's starting to shrink in the belly department.
TBTB: He is gravely ill, my guess is. Fish do not stick out their heads out of the water like you describe because of competition or hunger or for any good reason. He is probably instinctively looking for a way out.
Please, consider everything you could tell us according to this MFK-recommended list:
http://www.monsterfishkeepers.com/forums/threads/fish-health-diagnostics-read-before-posting.210102/