There is no minimum balance required after the initial $10 deposit. However, don’t forget that the interest rate you receive is dependent on your account balance!
Sometimes more is more! (Side note: if your account is at $0 balance for several months, you may receive a friendly email asking if you are still interested in keeping the account open. Additionally, accounts with no customer initiated activity for 12 consecutive months will receive a notice before they reach what's called, "dormant account" status, and a $5 monthly fee will apply if the account remains inactive - this is for account security, not to be mean!).