You should avoid all artificial sweeteners. Some, like Aspartame can be dangerous. It is an excitotoxin that can cause numerous neurological problems including mimicking the symptoms of MS. There are many excellent articles on the Web about the dangers of this product. For a quick overview of all the common artificial sweeteners you can go to Mercola.com and easily find information.
Some people recommend the herbal sweetener Stevia. It does give a pleasant taste to teas and has no calories. The down side is that very little is accomplished by feeding the sweet tooth even with calorie free products. The great strength of fasting is that it can help us change our food addictions and eating habits while detoxing the body. Feeding the body's craving for the sweet taste does not help us overcome that addiction, besides it continues to feed toxic substances into a cleansing body.
During this time of a temporary radical change in diet you should try to overcome your cravings for sweets by learning to drink your teas unsweetened. Give up caffeine, refined sugar, Sugar substitutes and other unhealthy foods before you begin to fast for an extended period. After the fast it will be easy to adapt a new pattern of eating and not go back to the habits that caused weight gain and other health issues in the first place.