The white build-up is caused by calcium deposits or minerals found in water. You can use vinegar to remove the build-up. If you don't have vinegar, you could also use lemon juice, although vinegar works best. Just put some on a clean cloth and rub over the calcium deposit. Rinse with warm water when you are finished. If you’re setting up an old aquarium, you will also need to clean the aquarium hood and possibly the light strip. In addition, the light strip will need to be tested. If the bulb is black on both ends, you will need to replace it.