Reef stores !?!
You are going to Costa Rica, and looking for a pet shop!?!
My recommendation is to do as much diving as possible.
My experience was on the West coast of the Guanacaste region. Without question, the absolute most biodiversity I've seen on a dive. Not the greatest corals, but overwhelming amounts of Fish/Sharks/Turtles/Eels.
I hope you have a great trip!