My RES, Spot, is a two-year old male. Until about 2 months ago he was strictly an indoor turtle. He lived in a 50 gallon tank without a filter. The filters I tried when he was a youngster just seemed dirty to me, so I got in the habit of completely changing his water and rinsing the tank clean twice a week. Never missed a change.
In June I decided he'd be happier outside. Until I can build him a "real" pond, he's living in a plastic, blow-up kiddie pool, diameter 5 1/2 feet. I keep the water around 15 inches deep. Spot has a basking platform in the middle of the pool built from cement pavers, stairstepped see he can get up easily. There are trees around the pond but part of it receives direct sun for several hours every day. Until 10 days ago I changed the water completely about every 5 days. He seems very happy to me.
I've got two questions.

One - I've got a water quality testing kit and after 10 days, the tests say the water is amonia, nitrite and nitrate free. The ph is high - over 8. The water looks crystal clear and doesn't stink. There's dark green algae on the bottom. We're suffering a severe drought in Texas and I don't want to change water wastefully if I don't need to. Can I trust that the water quality is ok and I don't need to change it unless the quality tests give bad readings or the water starts looking dangerous?
The other question is this.

When winter comes I'll have to bring him back inside to his "little" 50 gallon tank. Is this very traumatic for a turtle that's used to more freedom and the outdoor environment? Is there some way to make the transition easier for him?