Angelfish problem

Hi

I have a 140 litre (37 US Gallon) tank, with external eheim filtration,
which contains:

3 Angelfish (2 male, 1 female) - the larger male is about 3 inches from nose
to tale, whist the other 2 are probably 2 1/2 inches.
6 neon tetras
2 corydoras, around 1 1/2 inches long
2 loaches around 2 inches long
1 red-tip shark (which isn't really a shark!) around 2 1/2 inches long.

The tank has a sand bottom and is well planted.Water quality is good and I
do a 15% water change every 2 weeks. The tank has been established for
around 18 months with all the afore mentioned fish.

The larger male Angle has recently developed a list and when stationary
leans at about 45 degrees, sometimes more. When swimming he is normally
just about upright. He has no other symptoms or marks and appears to have a
healthy appetite. Any thoughts as to what might be causing his condition,
and how to treat it, would be much appreciated.

Regards

Neil Cummins