I have never seen a fish tank in houses for sale (including model homes for sale by builders). Leave alone built-in tanks. Is that because I have not seen enough homes or California homeowners do not like fishes?
A friend (in Orange County) bought a home where the previous owners had installed a large, beautiful, salt-water tank built into the wall separating the family room and living room. It held a lion fish, a clown fish, and a handful of others. I think they hired someone to regularly maintain it. Sadly, when they had some work done on the home (paint?), the fish died. Afterwards, they removed the built-in and ended up filling the wall back in.
Have you seen that TV series Tanked? Fun show to watch if you like aquarium.
The reason not many will do a build-in probably due to cost and space constrain. For salt water tank, besides the main large build-in display tank, you will need the space for the plumbing, filter pump, and may be another support tank hidden in another room.