Unless there is a longer term agreement, then a 30 day notice is fine. I also agree with giving them more though, at least 45 days.
I would then prepare a reminder notice for all those that continue to pay the old rate after the effective date. I would NOT chanrge a late charge for the first time. But I would state in the reminder something like:
Thank you for your recent payment on Storage Unit xxx. As per our letter of (45 days previously) we raised the rental on your storage unit to $xxx. So you owe an additional $xxx. Please remit this amount ASAP, but no later then your next month. If the correct payment isn't received you may be subject to late charges.