How can I use old hot tub water in yard?

to NGA

Is the water chlorinated? You can get the hot tub water super hot and hope most of the chlorine dissipates. There is a chemical you can get to remove chlorine from tap water but hot tubs have a little more chlorine then tap water. It might take a lot.

If you run the bubbler in your hot tub for a while, that usually blows off a lot of chlorine. Use your test kit to see how much chlorine is in the water, and when it gets very low, then let it cool off some, and pump it out onto the garden.

We used to backwash our pool filter onto our garden beds in Utah with no problems at all.