You may have to replace your bulb. Bulbs in UV’s have an effective usage life of 9 to 12 months. If you unplugged your UV during the winter months you extended it’s life. As a rule of thumb, a UV’s lamp should be replaced every year. The bulb loses power as it burns, therefore, it may still be lit but operating at 20% of its original power, rendering it ineffective for the purpose of killing algae. You’ll know when you need to replace your lamp simply by noticing that your UV is no longer clearing up the free-flowing algae.