It is impossible to keep delicate shells fresh and clean at the bottom of an aquarium, for they quickly become covered with a green aquatic growth that defies all efforts to be scrubbed oft. The shells may be cleaned by plunging them in a boiling mixture of 1 part of hydrochloric acid to 10 parts of water Hold them with wooden tongs, and remove alter one second to clean cold water. Repeat the operation it necessary, but if the shells remain in the acid beyond the prescribed time they will be eaten in holes, it not altogether dissolved. If the shells are to be replaced in the aquarium, it is not worth while to clean them repeatedly. Introduce a few fresh-water snails into the aquarium, and they will keep down the green growth.