It is OK gaining some weight - dissolving in water old hardened stuff.
Water may taste horrible when the mouth has it own share of toxins circulating, so the water dissolves some of them and this is what gives the bad taste.
One of my relatives had problem with drinking water at any time, because she was hugely toxic and there wasn't any water that tasted OK for her in her entire life. ... good that this happens to you only in fasting mode.

toxic taste is what I call it. It will get better after some time.
I like to went nuts from a toxic water taste when I did my last long term water fast. It was sweet tasting about day 15. I could hardly handle it. I was suggested by someone that does these a lot to take 1/8 tsp of Sea Salt to one gallon of water.
Since then I know what caused the sweet taste. I took a medicine for GERD prevacid I think was the name that gave me a sweet taste when I still had acid reflux even with that medicine about 8 years before the actual water fast.
Tongue probably is coated. I scrape my tongue and wash my mouth with water very well. I don't use anything because I want all toxins to leave.