tub that holds about 25 to 35 gallons of water (which is pretty typical), then you can use up to about 1 to 1.5 cups of 35% peroxide in the bath and then feed the water to your plants. if you use less peroxide than this, that’s totally fine for your plants. less is always okay.
if you use more peroxide than that, you have a stronger water-peroxide mixture than what’s recommended for watering plants. what’s too strong is open to discussion (or experiment). what you can do is add additional water to the tub (after you are done bathing) to further dilute the peroxide.
if your tub is a lot bigger 