Pescadorita on Humboldt is pretty good. (It is owned by the same people that run La Dorita steakhouses)
Some great seafood risottos and decent Paellas.
They also have some daily specials which will almost always have good fresh fish from the ocean or some river somewhere in the country.
One of the reasons for the lack of good fish restaurants is that probably 70% of the catch is exported in factory ships to Spain and China.
So as a result, the wholesale route to market is actually quite small. - in logistic terms the biggest seaport which lands fish is more than 4 hours from the capital.
Hake , squid and shrimp are the big items . Any salmon you see probably comes from Chile