automatic fill and drain system. A valve opens and puts in tap water
everyday (15%) And the overflow tube waters her garden. She uses no
declor. Is it possible to do this? She has done this for 3 years

Is it possible? Sounds obvious to me.

Yes, but this much of a water infusion daily, in a region that happens
to use chloramines, might be a bit dangerous. Because water districts
can change to chloramines on a whim, or even temporarily, it can happen
to the water gardener at any time.

A smaller percentage, though, should be safe in any case. To do math,
where it takes chlorine a day or so to evap, chloramine is closer to
a week. 15% daily infusion of chloramine purified water, would invite
a noticeable level in the pond.

5% perhaps?

C//