Bristlenose Laying On It's Back

[jonbrown]

The past couple of mornings when I have looked in my tank my Bristlenose has been laying on his back on the bottom of the tank. He has a favoured piece of bogwood which he normally hangs upside-down on, but in the mornings it's almost as though he has fallen off. Had a good look when he's stuck on the glass and seems ok, although recently has looked as though he's only half stuck when on the glass,........ hopefully that makes sense.
 
Seems to be eating ok (algea wafers and courget mainly) and water parameters are fine and oxigintaed with two airblocks for 16 hours a day. I don't have any other bottom dwelling fish so can't imagine he's getting hassled or having excessive competition for food.
 
I don't exactly know how old he is, other than he was about 4 inches long when I bought him and is about 5inches long now, which must mean he's about full size.
 
 
Is he just getting old, or any ideas would be welcome.

 

[fluttermoth]

What are the actual numbers from your tests?
 
Does the tank have a filter, apart from the airstones?
 
What temperature is the tank, how big is it and what other fish are in it?

 

[jonbrown]

I have nil ammonia, nil nitrite and nil nitrate, ph7.5
 
The tank is 275 litre with an eheim pro 2123 external filter, water temp 26degrees.
 
Other fish include 11 rummy nose tetra, 5 lemon tetra, 5 cardinal tetra, 3 beckfordi pencil fish, 2 halfbeaks and 2 small angel fish