To begin with, African violets prefer soils that are evenly moist. Improper watering, especially overwatering, is a primary cause of problems. Allow the soil to dry out only slightly before watering, then water from below. Flush out fertilizer salts with a thorough watering from the top at least once a month. And keep wet foliage out of the sun as the heat can leave marks.
Violets are sensitive to extremes in water temperature.
Water that's too hot or too cold will cause white rings on the leaves. Allow water to stand overnight to bring it to room temperature and dissipate any chlorine present. To produce constant bloom, apply fertilizer on a regular basis. But don't over do it. Slow- release fertilizers are a practical way to supply safe levels of nutrients at each watering.