Kai are easy and long lived, which is why they are popular. Being in LA your options are a little larger than some:
Anything from the Gold Fish Family (comets, GF, Fancy GF etc..)
Pirtch and Sunfish are also nice but not "non agressive"
A massive school of minnows?
Any Cold water fish will do well.
I have a single blue gill in mine with about 200 minnows and it works well.