So when is it to cold for hot tubbing?  Today I went to do some mid-weeks on hot tubs.  As soon as my hands left the spa they instantly lose felling.  And you can forget about filling a spa or rinsing off a filter with a water hose.  They are long green ice cycles curled in a unbendable coil.  For the one spa that was significantly low on water I pulled back the pool cover.  Broke the thin layer of ice on the pool surface, and used a 5 gallon bucket to transfer water into the hot tub.  What ever works in 25 degrees right.