In my experience...Some barbel do not recover fully even after resting up in the net for longer periods. With unseen gassed up fish, they will go off seemingly ok, but as they swim deeper to get to the bottom, the trapped air pressure inside forces them back to the surface (just like a balloon in water), to where they turn belly up, float downstream and usually die, which is usually unseen by the captor. Gassed up fish are usually caused by bringing/playing the fish up from deep water to quickly or by having it out of the water too long, but there may be other reasons too. In the recovery net, barbel have to regain their balance and stabilise themselves, as they can be disorientated. If they tilt over on their side or on their back, they are not ready to go. A good tip is to monitor and check their 'Eyes' to see if each one moves 'downwards' and back and forth when you tilt the barbel's body slightly side to side. If the eyes don't move and are 'fixed' when tilting, then the barbel has not recovered and is not ready for release. You can do this in the water/net. As mentioned before, the barbel can seemingly go off ok, but they can instinctively go under weedbeds for cover or in deeper slacks to rest up and recover further. Here, on some occassions, they can still turn belly up on the bottom and out of sight, unseen by the angler. Always best to keep looking out after release (with your landing net handy) to see if the barbel re-surfaces or is in trouble. As Graham say's, rubbing the barbel's belly can help to get rid of some of the air, but sometimes not all of it, which can be an unseen problem after releasing.