An apparatus for treating a liquid includes a container containing a liquid, and a magnetic treatment device therefor. The magnetic treatment device may include an elastic band removably fastened around the container and urged thereagainst by elastic deformation of the elastic band, the elastic band being deformable to accommodate containers having different sizes. The magnetic treatment device may also include at least one permanent magnet carried by the elastic band for magnetically treating the liquid in the container. Alternately, the magnetic treatment device may include a band with a joint therein or a drawstring removably fastened around the container.