my guess is the dog got him, but once before i had a fish jump out and i could not find it forever, but then one day, i found him im a window screen track of all places.
Last two times I heard of anybody check a dogs poop was in the Nick Cage/Angelina Jolie version of "Gone in Sixty Seconds" and Cheech&Chong's "Up in Smoke".
Well my guess is that it jumped out and somehow plastered itself to a surface somewere....then dried up. I once had a Tilapia do that.... Checked everywhere and couldnt find it.... until a few days later a trail of ants showed it to me.... 15 feet away from the tank and underneath the living room sofa. Poor guy...he really fought hard for life.