I was about to suggest a similar/the same suggestion as Miss Dorsal - basically, it will be a lot less distressing for the fish if you kept him in shallower water ie not so far for him to swim or roll up/down. It's probably best to separate him and put him in a spare tank to do this, perhaps transferring at least some of the established tank water across into it. If you have a spare filter, then put some of the media (eg sponge) from the main tank filter into it, and put just enough water in that tank to cover the filter.
If you don't have a spare tank, then EITHER you could lower the water in the main tank until it just covers the filter (provided that this doesn't significantly reduce the overall litres in the tank for all the fish) OR a plastic tub as Miss Dorsal describes is a good, alternative, cheaper option. I'd advise, though, that you use one that is quite big - I'm not sure what to suggest as the minimum size but I think it would have to be a minimum of 20L, with daily water changes. (I tried this once with a fantail in a 10L tub and lost him on the first evening - I think that size may have been too small.)