That's a groovy lookin' tank MartinB! I gotta agree the Dr. ( Oizo ) though, the tubing attracts the eye immediately and looks a bit out of place.
If it were my magnificent tank ( which it isn't, so feel free to ignore this one ), I'd just put the air stone right into the corner underneath the heater. ( I think that's the heater in the back left corner ? ) Will keep that tubing out of the way, and also help disperse the heat through the water.
All in all - great looking setup. Well done!