They go deeper when it's hot because the deeper water holds more oxygen.

Sandy, I love your idea of turning the pool into a fish pond. With the deck built around it the landscaping possibilities are endless. I envision lily pads, clumps of various grasses, vines, a waterfall, hidden lights . . . an oasis in your backyard.

I would probably start with the external landscaping and let the pool freeze over winter to get rid of the CYA. Mine ends the season with around 60 ppm and is at zero when it's finally thawed out the end of April. To the best of my knowledge all 48" or so freeze solid. We don't have a deep end.

But that would mean continuing the chlorine maintenance for the rest of this season and you probably don't want to do that.