The Nanite Tank is a large device at the Paradise Base in the Arctic, where Rex Salazar unloads any excess nanites absorbed from the EVOs he has confronted. The tank holds every single excess nanite from Rex, and the tank itself has steam generating soles with an unknown purpose as of now, but presumably to relieve pressure.
When Rex arrived at Paradise Base to deposit the excess nanites he absorbed from the EVOs he cured. Weaver, the head technician and scientist at the base, aided Rex in the process of depositing the nanites. When it was discovered that Weaver was reactivating and selling the extracted nanites to Van Kleiss, Weaver tried to scare Rex and his team into giving him safe passage with steam from the Nanite Tank. He overdid it and he was infused with every nanite from the tank, transforming him into a massive and grotesque EVO which Rex tried and failed to cure. Holiday defeated it by ramming their transport into the EVO Rex was then rushed to Purgatory Base where the nanites he absorbed from Weaver were deposited.
The Nanite Tank later played a critical role in Van Kleiss's defeat when Rex refilled himself with nanites to activate the Control Nanite, restoring his powers and giving him new ones. As a precaution, the nanites in the tank were wiped clean of their programming after the incident at Paradise.
Again, it was shown at another base to off load Rex's excess nanites. He must've used this one since he was no longer part of Providence. However, it had a striking resemblance to Purgatory. After he off-loaded his nanites, he reprogramed his nanites to activate a shield around the nanite tank to trap NoFace, Hunter Cain, Gatlocke and Valve.