A coated tongue reflects the inner condition of your digestive tract, and which in essence means that there is some fermentation/putrefaction within your digestive system.
Try and eat a properly combined diet and ONLY WHEN HUNGRY and your coated tongue will disappear while eating, in a relatively short time-frame.
A water-only-fast-conducted according to thirst, and thirst only, will usually make a coated tongue worse initially, as toxins are discarded for elimination, but will clear as toxemia is lowered or eliminated.
A non-coated tongue on a fast will mean that toxemic levels are already low, and the work of the body in eliminating toxins is almost there.