Letang, who was activated from long-term injured reserve earlier in the day, finished with two goals and two assists.
Aaron Ekblad had a goal and two assists, Sam Reinhart and Matthew Tkachuk a goal and an assist apiece and Colin White one goal.
Fiala also had an assist, while Anze Kopitar scored two goals and Samuel Fagemo contributed one goal for the Kings.
Avalanche 3, Capitals 2Artturi Lehkonen, Alex Newhook and Andrew Cogliano scored goals, Alexandar Georgiev had 37 saves and Colorado beat Washington in Denver.
Logan O'Connor had two assists for Colorado, which has won six straight despite Cale Makar missing his fourth straight game with an upper-body injury.