coral in FW

Yes it will infact some use coral to buffer there PH. This can be a good thing or a bad thing depending on your needs.

In Los Angeles, maybe, not. If your pH is already 7.8 or higher, you might not see much change.

If you mean to use coral in the tank itself, as a decoration...this is not a good idea. Beyond the pH/hardness issue, coral will likely present danger to freshwater fish who are not used to it and can be injured.

Even scoria lava rock can be a little dangerous to your fish. I notice tiny little scrapes on them quite often in the tanks where I have it. Nothing dangerous, but still....it has caused me to remove a good amount of it.