From what I have gathered reading all the sick indoor bonsai posts on this forum, I would say your problem is due to the method and frequency of your watering. It is best to water not to a schedule but only when your plant needs water. This can be determined by checking the soil or by the weight of the pot. When your plant needs to be watered, water thoroughly from above with a watering can or the spray nozzle on your kitchen sink. Submerging the pot in a basin of water can lead to an accumulation of salts in the soil that is harmful to your plant. Also misting to frequently can lead to overwatering from the run off accumulating in the soil. I would recommend that you water once thoroughly and then leave it alone for a while. Wait until the plant is on the dry side and then repeat the process. The key is not to water more than necessary and when you do water, water from above and do it thoroughly.