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There should be valves to redirect the water to and from the spa. Ken
If you leave them open for a very long time or add other chemicals to it then yes they can sometimes go bad but most spa chemicals are good
The same as you would a fresh water spa.
No spa water is not pure there are other things added to the water
The check valve between the spa and the pool is defective.
Yes, especially after you leave the cover off for a few days until you get a zero sanitizer reading.
There is no time limit on SPA Tasks.
I would NOT do that unless I were certain that the spa was built and designed to handle the change. Salt water can do serious damage to components that were not designed to withstand it.
The water should not be turned off until at the recommended level. Leave the hose on. Pool & Spa
When you have a spa used by the public you want to drain it often. The water in the spa should always be clear and clean.
Try the Oxygen-Ion Turbo Spa Massager Whirlpool Device JTM-302. You may want to try some scented oils in the water and make sure it is hot.
It depends on what was in the spa. If was just hot water, then it should be fine. If it was dirty water, monitor the dog carefully for any signs of sickness.