Hair fall and Hair thinning both are very separate things and needs to be evaluated. Change in water can give you hair fall which can brought under control with proper treatment and hair care. Hair thinning is many a times irreversible and needs to be treated medically. Other than change of water we need to find out if there are any other underlying causes. And if so needs to be treated. My advice will be do not wait too long to start with your treatment if you feel thinning has Started then treat it medically. Homeopathy medicine are safe and have been very effective in treating Hairfall and hair thinning.
Next Steps
You can consult me on practo, I have 12 years of experience in Homeopathy and especially Hair related problems.
Health Tips
If water is hard then you can use RO filter, or you can use drinking water to wash your hair. You can squeeze lime in bucket full of water that also helps to reduce the hardness of water. Do not use very hot water to rinse your scalp. Also you can include some multivitamins.