Kiwi8,
I agree entirely with Naturalist here.
Hang in there and you will feel better and stronger before long.
The weakness is caused by toxins being released into the bloodstream where they can then be eliminated thru your kidneys lungs and skin, which of course are all organs of elimination.
The tongue clearing is one of the signs that elimination is nearing the end, although I am not sure how long you have been water fasting.
I felt lethargic and weak for at least the first 16 days of my first prolonged fast.
Just hang on and the benefits of post-fast will far outweigh the "suffering" you are experiencing now.