how can I clean my waterwise distiller??

I have been using my waterwise 4000 distiller for about 2 months now and I have noticed some accumalation in the water tank which is extremely hard to remove with water and a sponge. Usually when I make a gallon of water, afterwards I clean the tank out with water. Now the debris inside the tank is getting really hard to remove with just water. Does anyone know of any way to get that accumalated gunk off the bottom of the water tank?? Also I want to know is it harmfull to make distilled water when you have that accumalated debris at the bottom of the water tank??