I would have thought that, as it's a very smooth surfae, it would be no problem to the fish and it's scales. The safest thing, I think, would be to make sure the inside of the bag is wet. I know this may add a very slight amount of weight to the final reading (a problem if you're going down into drams) but otherwise I wouldn't have thought it mattered.
I can honestly say that I weigh all my fish using a carrier bag and have never caused any scale loss that I know of, none being left in the bag after weighing.
I would go with the carrier bag. As long as it is clean on the inside it will do no damage to the fish. Some of the cloths shops bags are best as they tend to be wider at the base supporting the fish better.
Thanks guys. A thought has occurred to me since, a mate suggested using a Persil soap tablet bag filled with scraps of fish to attract Pike when deadbaiting. Maybe the same sort of bag could be used for weighing small fish,if they still make em.
To small with to large a holes the fins will get caught and damaged. If you need to weigh fish that small nip down to your local jewelers and ask for one of the bags.
I so a angler once use a set of kitchen scales with a long sandwich box on for weighing small fish.
After a long search I found some scales that measure in divisions of 1/4 oz so no problem there its just the container aspect.
Will try a jewellers in fact I'll anything reasonable. May even cut up my wifes tights but not while shes wearing them.