The below minimum balance fee you saw on your savings account is most likely what is actually called the "monthly maintenance" fee. The reason why you were charged this fee is probably because the balance on your account went below the minimum required to waive it.

Many banks and credit unions charge a fee to make up for the cost of maintaining your savings account. The exact value of the fee varies between banks and even savings accounts under the same company.

That being said, there are ways to waive the maintenance fee. The most common ones are:

Link to a checking account from the same bank;
Maintain a minimum balance of a certain amount;
Post an incoming transfer of a certain amount.

Ultimately, the best way to find out how you can waive the monthly maintenance fee is to check your bank's website or the documentation that you received after opening your account.