https://www.theverge.com/2018/12/17/18144120/fitbit-os-3-ionic-versa-update-couch-to-5k-apps

Fitbit is updating its Ionic and Versa watches today with the latest version of Fitbit OS, which adds support for a handful of new third-party apps. Fitbit OS 3.0 will also let users quickly log their water intake, sleep, and weight straight from the watch instead of through the mobile app.

Of the new third-party apps, some of the more notable are Couch to 5K and Achu Health. Couch to 5K is a popular training program that coaches beginning runners toward completing a long-distance race, and the app can display daily exercise goals on the watch. Achu Health measures your body for unusual temperature spikes unrelated to exercise to predict potential illnesses. Other partner apps will arrive next year, including TRX, Gold’s Gym, and Mindbody.