I didn't understand if you do through each line in the DB,
if so, why not select all records that have "WHERE balance < 200"?
and as Jibesh suggested if you need update, that can also be done in one action.
if you need help with threads in order to perform an action on each row of the result
you can check out Sacha's article:
Beginners Guide to Threading in .NET: Part 1 of n