Unless all of it is Ca or Mg, you'll be able to grow lots of stuff in that water. (You should be able to get Ca and Mg ppm (mg/L) from your city's water quality report.) SCAPE is (I guess was, but some folks still communicate or hang out) a club with multiple guys who have placed in AGA and plenty of regular folks who can
grow plants, and more than a few of them dump GH booster into Southern California's hard (but less hard than your) water, for example.
Be aware that some folks feel Ca and Mg should be in a ratio to one another. Many folks that can grow practically whatever they want agree some plants simply do better in soft (extremely low KH, usually less than 4 dGH) water, and a lot of those guys don't think those particular plants are cool enough to warrant their mixing or reconstituting RO/DI/etc. I think the majority of experienced folks agree Ca and Mg, assuming they are available, are well down the line of macros (C/CO2 is a macro) and Fe (also as a proxy for "comprehensive trace" levels) when growing a tank.
But you may have read all that already, so here's two cents: I kept a 0KH / varying Ca targets / ~10ppm Mg tank for a while, and plants grew in ways I had not seen before, and I was finally able to grow certain plants I could not grow before. Still, in my weaksauce experiments with Ca in my weaksauce tanks I don't think it really matters. I do think low KH simply grows nicer plants. I still use L.A. tap because reconstituting and more importantly using/getting RO and or DI sucks.
I'll bet a lot of guys with world class/competition tanks have never even looked at Ca outside of maybe GH Booster because they never had to.
I hope this helps.
