Barbel recover differently in different rivers even different stretches, on the upper Lea it is almost impossible to rest them as they kick off strong every time whereas much further down the river they need much more recovery, that's the trouble with D.O. and overall water quality in that you just can't tell by looking at the water before you fish.
Use common sense whatever you are fishing for, think can I return a fish safe, preferably getting in the water with it, is the water too stagnant in the margins where I am for it to recover, or too fast, sometimes well oxygenated water can be bombing through and no place for returning fish.
I had a few of Barbel on the Severn last week whilst after roach, often you can't avoid them and because of this my gear was a compromise, much too heavy for roach just in case, bottom line is I took way longer than necessary to rest them in the net in the water, before and after and unhooking in the water if possible, folk on the other bank were probably wondering what I was doing but I didn't care, take as long as possible to rest any fish, you're down there long enough after all. :thumbs: