I am glad you are working with a veterinarian. There is nothing wrong with prescription food. Check with the vet that has examined the dogs before making any diet changes.
I like Fromm Classic Adult as a base, also I would consider Purina Pro Plan Focus Salmon for sensitive skin and stomach.
I feed 2 meals per day, add a splash of water or plain chicken broth and a topper.
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I feed 1/3 cup of kibble as a base twice a day for a 20 lb dog. An occasional snack, maybe 1/2 raw carrot to chew on.
Check their teeth as they may need professional cleanings. If all is okay, start brushing their teeth twice a day, I like Petrodex (6.2oz tube) is economical. Google to find it.
Start walking the dogs on hard pavement for at least 10 minutes at a time, work up to 1 hour a day. This will help their legs. You will see a big difference after a few months.
For science based veterinary information go here, use the search engine to look up specific topics, ie: nutrition http://skeptvet.com/Blog/
