A device for dispensing CO.sub.2, that is to say carbon or carbon dioxide, which can be fitted in aquariums or containers for holding live fish, comprising a dispenser casing (11) or body, equipped with a mixing chamber (15) into which a flow of water is delivered by a pump (12) and a flow of CO.sub.2 from an infeed duct (18).The mixing chamber (15) being bordered by at least one filtering element (13) which occupies the lower half of the dispenser body (11), closed at the front by a mesh cover (14).