Dear Mr. Kolb:
Thank you for writing back to me. I hope that you and your family are well. You can put the anti freeze in, but it might be better to maybe just blow some air from a wet vac into the inlet pipe to get any remaining water out. If the heater is level then I doubt that you would have a problem but I like to be extra careful as if ice damages the heater all the manufacturers will act like they don't know you. you can always call the factory to get the official scoop. 800 447 9667. Then if you follow their exact words they can never say anything. Each manufacturer has their own system or preferences. Some people also put a custom made cover on the heater in the winter too keep the leaves and the weather out. Here is a link top some nice ones. I hope that you have a great off season and I will see you in the spring. Still waiting for the 4th of July pool party invitations too.
I hope that I answered all your questions. Please let me know if you have other questions or if you want me to call you. We are here for you.