Since you haven't already disclosed this information, you need to provide notice 30 days before making the change. The notice has to say that the accounts will be subject to a $3 fee each [month/statement period] if the balance falls below $100 at any time during that period.
Or, if your fee works differently, explain how it works.
Be sure to be consistent in how you label the fee in the notice and on the statement.
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John S. Burnett
BankersOnline.com
Fighting for Compliance since 1976
Bankers' Threads User #8