Active Aqua Hydroponic Water Chiller, 0.25 HP, Pure Titanium, 40-92 Gallon

This chiller comes from hydroponics, where it keeps nutrient reservoirs cool, but its listing names cold plunge baths as a use and rates it for 40 to 92 gal (151–348 L). That makes it a fit for a small upright tub rather than a long, lie-down one. It has a titanium evaporator, a digital temperature control and a Boost function for faster cooling.

It is a bare chiller: water has to be pushed through it by a separate pump of 396 to 925 GPH, and neither pump nor hoses are in the box. The listing also leaves out the lowest temperature it reaches and how long a cool-down takes, so those are questions for the manual. At 19 in (48 cm) tall and 48.2 lb (21.9 kg), it stands on the floor beside the tub.

Tip: Size the tub to the chiller, not the other way round – a tub whose listed capacity sits inside the 40 to 92 gal rating keeps the chiller within what it was built for.