One of my pets had its Treasure increased by Pet Zapper V2. Are there any other ways to increase it? I have pets I don't want to zap.

you can buy Magical Keepsake Boxes from the Millilonare Center or user shops. They will increase the treasure spots 1-3 spaces

Oh, thank-you-thank-you!

You can also purchase a Multi-Compartment Keepsake Box from the Cash Shop for 500 credits, which gives your pet's treasure a minimum of 10 spaces. Not sure what the maximum is.