[DoctorBeaver (OP)]

I've been watching our fish. It's slightly preferable to watching the paint dry. While indulging in this mind-numbing kill-time I noticed that 2 of them were vertical, stationary in the water, with their noses upwards. How do they do that?

I think they use their swim bladders to adjust their depth, so is that something to do with it?

[RD]

air bladder = gas bladder, the preferable term since the composition of gases may not be identical to that of air (a thin membranous, sometimes alveolated sac in the dorsal portion of the abdominal cavity. Contains a varying mixture of gases and may be one, two or three chambered. May be connected to the gut by a tube, the ductus pneumaticus (then called physostomous) or unconnected (then called physoclistous). May function as one or more of:- hydrostatic organ, sound producing organ, sound receptor, respiratory organ. Found in Actinopterygii. Often lacking in bottom fishes. Sometimes called swim bladder, a less appropriate term).

http://www.briancoad.com/Dictionary/DicPics/air%20bladder2.htm

If the fish can control the extent to which each chamber of the bladder is filled with gas
 this could enable the fish to change its attitude in the water.

[DoctorBeaver (OP)]

Ah, I didn't realise it was multi-chambered. Thank you.

[LeeE]

I think that non-bottom dwelling fish must be very finely balanced if they are not to expend lots of energy maintaining their attitude/orientation.  If this is so, then it would not take a lot of energy for them to maintain other attitudes.