You could use bleach, if it is extremely dirty. A shower is a great way to clean all the loose dirt off. I prefer to use vinegar, which is better at removing any hard water stains. NEVER use hot water to clean it. Fill it up with warm water and plenty of white vinegar. Leave it for a day or two, use a glass scraper if necessary. It should come up like new. Bleaching or using vinegar is harmless, as long as you give it a good rinse in warm water....