I'd say something like: "We're so sorry to hear that you weren't pleased with your new apartment home, as resident happiness here is a top priority! During the two tours that we gave you prior to move in, we thought that everything was in order and that you had sufficiently viewed the space and asked any questions that you may have had to our staff. If there were any issues directly after move in, as with all of our new residents, these could have been shared on your Inventory and Condition form for them to be either noted in your file or if necessary, corrected immediately by our maintenance team. If you're still currently experiencing any issues, please contact our Leasing Office ASAP so that we can discuss and resolve them. We're here from Monday-Friday from 8-5pm and on Saturdays from 9-4pm. We're looking forward to speaking with you in more detail soon."
Other renters will see that you responded in a kind and helpful way and it shouldn't deter them from checking out your homes.