Thanks all of you, it is wonderful to have found you! I now have Pete (and Lily, too) on a rice and chicken diet, and will add omega 3 fats. Sweet potatoes were sneered at for some reason, and they have never liked fish much (tuna is occasionally accepted).
Lily looks so much like the photo of the Tanza tri in your post, Pat F. She is such a comedian! And I agree, my gut tells me to try the diet and medication route first while working with my Vet, but it is tempting to want to find out what is going on…although I wouldn't opt for surgery at his age and the ultrasound, etc will be expensive. Pete is such a cuddle-bug! He doesn't appear to be in pain but I do believe he feels slightly "sick to his stomach". And he yearns for his evening treat which I stopped when his diarrhea got worse (and why during the night?). Any thoughts about treats which might be tolerated? Both Pete and Lily loved pig ear strips more than anything else in life, I think.
I appreciate all your thoughts, replies, and advice.