Puppy chest rash: Is it diet, environment, and what are remedies?

I'm sorry to hear that Hatchi has developed this skin lesion. Possible causes include a bacterial skin infection (most commonly caused by underlying allergies), ringworm, mange, or simply irritation from the sand outside. Without diagnostic testing at the vet's office, it is impossible to definitively determine the cause of this lesion. However, given his recent changes, you can try a few things at home to try to resolve the issue with a bit of trial and error. For starters, try to return Hatchi to his normal diet, but be sure to do so gradually by mixing the foods together over 5-7 days to decrease the likelihood of causing gastrointestinal upset. Try to keep him indoors, particularly while this lesion is healing. If possible, try to eliminate sand from his normal habitat since this can be quite abrasive. While indoors, leave his collar off to prevent moisture from building up under the collar and clean the collar well. You can also try bathing him in a medicated shampoo (this is one example: http://www.drsfostersmith.com/product/prod_display.cfm?pcatid=27982). Make sure that Hatchi is receiving a high quality flea and tick preventative, such as Nexgard, Bravecto or K9 Advantix II regularly, even though you haven't had any noticeable issues. If after all of these changes the skin lesions persist or worsen, Hatchi will need to go to his vet for further evaluation. I hope that all goes well!