If your outdoor hot tub receives a lot of sunlight, you may have occasional appearance of algae.  HTH Anti-Algae will prevent the development of algae in your spa. 

Directions
Start the filtration system and pour 2 capfuls of HTH Spa AntiAlgae per 1000 l of water directly into the Spa, once a week.  If the water is infested with algae, double the dose.

Note
If your spa is not exposed to sunlight (e.g. the cover is on most of the time), your green water may not be Algae, but rather due to bacteria.  In that case, you should deep-clean your spa with a product like Spa System Flush, drain the spa, and refill with fresh water.

Volume
1 litre