Does anyone know a way to maintain an in-ground hot tub that is not in use? I know it needs to be filled with water in order to keep the tub from popping out of the ground, but the water becomes stagnant.

 

Tips for shutting down an inground hot tub:

"Beware of Hydrostatic Ground Pressure: Inground spas are not left empty in areas where hydrostatic ground pressure may pop an empty inground spa out of the ground. Place vinyl or black poly sheeting, at least 6 mils/15 mm thick, in the bottom of the spa to form a liner. Fill the spa with water to the bottom of the skimmer. This liner equalizes pressure and keeps water out of the pipes."

http://apsp.org/184/index.aspx

http://www.spadepot.com/spacyclopedia/winter.htm

Unless running the pump and circulating the water, the water will become stagnant. Covering hot tub with winter cover will keep out debris and mosquitos.