Barrels are used to keep 4 fish in one inventory space. To do this, you need a barrel. Catfish only need a common barrel, which can be bought from any barreler. Flounder and Lobster need higher quality barrels. Both cost the same, 5 coins, but the higher quality can only be bought in the Fishing Guild, although they are available for free in Calabash. The common barrel is available at The Crossroads fish barreler. When the barrels are opened, they collapse and disappear, and all fish are removed at once. You cannot buy a barrel and put the fish in yourself. Only a barreler can barrel the fish for you.