You may need to get an RO/DI unit, but I think those can be pretty expensive.
I'm not sure about San Diego, but I believe pretty much all of the LA area get's it's water from the aquaduct, and salinity is an issue due to evaporation (hundreds of miles of uncovered water, through deserts and everything). It's also a problem in the great valley, there are salt horizons forming in the soil because of the high mineral content of the water.
Water is kinda a messed up issue in Southern California. :/
If you've had problems using RO/DI water, maybe try just cutting your tap water with RO water? it should come close to halving mineral content if you just do a 50:50 mix - that way you still get buffering and micronutrients and what not, but not the excessive amount that is already in your tap.