The water will move through the soil and come out the bottom drainage holes. Allow it to drain away. Do this several times to each plant. Epsom salts contains magnesium that will attach to the fertilizer salts built up in the soil and help wash them out of the soil. Cactus thrive if kept cool during the winter months.
Lower temperatures encourage the development of sturdy plants and stimulate flower bud development.
Most cactus do best at temperatures from 45-55 degrees Fahrenheit, however, they still require high light conditions during this period, so a cool bedroom with a south facing window is ideal.