My tenant left a few months ago and i gave them their deposit back less water/electric - final month. It has now come to light that they never paid the water for one year. Who is responsible for paying this??

I wouldn't worry about legalities! If you want a continued supply of water you pay the bill. All part and parcel of renting properties here!

They took away my water meter last week for failure to pay Febuary bill which is 108 baht

Why is the water meter still with you since there is no payment for 1 year ?

i'm going to pay it, just wanted to know the legal side for future reference