It's a real shame that it becomes apparent that many anglers have so little knowledge of unfamiliar angling scenarios. Treating a river grayling or brown trout exactly the same as a stillwater cyprinid is seriously risking having a dead fish on your hands.
For those that don't appear to know, the notion that there is a one size fits all version of perfect fish care is pie in the sky. I appreciate that some folks on here think that the highest levels of care are lavished on carp. At the same time, others find the time carpers have fish out of the water to be excessive. The reality as far as I'm concerned is that carp are the hardiest of all species. The time they can be kept out of the water, and returned with no apparent ill effects, is quite surprising, certainly way longer than I'd subject them to. In respect to time out of the water, I'd put grayling at the very top of the list (at least of the more common UK species) of least tolerant. On top of that, you find that certain species are definitely more fragile than others, especially in the warmer months. Barbel and Pike certainly don't like the warmer weather and higher water temperatures.
I try to treat all fish with respect, even those I'm not particularly keen on. However, that's an awful long way from suggesting that I treat all caught fish in exactly the same way. You might get away with treating a tench, bream, or even a barbel in winter, as you sometimes see carp treated. You almost certainly won't get away with treating a trout or grayling in the same manner.