I live about a million miles away from California but here are some suggestions from my own complaints:
1. Communication is key. Your phone number and email should be always available- best to start a business email. No matter what, always respond to the customer.
2. Try and have a good quarantine process, always hold fish before selling for at least a week. Embrace local breeders.
3. Try and make sure the shop is as clean as possible if it is a lfs. If it is online, make sure to try and take good pictures of fish you are selling and not google ones. No dead fish and good looking aquariums are what attracts most people. Try and have info for every fish and price on the aquarium itself. Also, have some dry food. Friendly employees.
Dunno what fish people like in California, but have some guppies, bristlenose cories, tetras and barbs in stock as well!
Good luck, already looks you are starting a shop with good goals in mind!