The inquiry was launched after police received information that raised concerns about the deaths at Springwater Lodge Care Home, Calverton.
But post-mortem examinations and extensive tests revealed all six died of natural causes.
The home expressed its "deepest condolences" to the affected families.
A spokesman for the home added: "We want to thank everyone in the community for their support for the home during this difficult time.
"Delivering the kindest possible care is at the heart of everything we do and we are grateful for the thoughtful words of our residents and their relatives.
"Their health, safety and well-being will always be our number one priority."
A force spokesman said: "Nottinghamshire Police continues to work closely with the home, the Care Quality Commission and other partners to ensure that appropriate safeguarding measures are in place."
The deaths of six residents at a care home - which sparked a murder investigation - were all due to natural causes, Nottinghamshire Police said.