During water fasting, your hair growth, and your nails growth will slow down, as it takes protein to grow hair and nails, and body is saving on proteins during the water only fast.
Now, hair and nails are already growing very slow, so it will be even slower during the second part of the water fast.
If you are fasting for a long time, longer than 20 or 30 days, hair growth may stop altogether once your fast extends longer than 20 days.
That is just natural program of human body trying to preserve limited resources (no proteins or minerals are consumed for a long time , so proteins and minerals are limited resources. )
Water fasting can also trigger temporary hair loss in some people.
Most likely due to toxins (heavy metals like mercury from Amalgam or led from old water pipes etc) being dumped into the hair.
In those cases, body dumps toxic hair, so there is a temporary hair loss like with chemotherapy, but it regrows later stronger.