Are both tenants your tenants? Are there other units available? If so, maybe offer to move the downstairs tenant to an upstairs unit so there won't be noise from above. If that won't work, maybe install carpet and thick padding in the upstairs unit.
I doubt you will be fined. If the tenant keeps complaining, but the complaint is not reasonable, there really is nothing you can do. Just keep a record of the complaint, notify the tenant in writing that you have investigated the complaint and found nothing wrong.
As a last resort or maybe a first resort if this is a problem tenant, offer to let the complaining tenant out of their lease early.