Phosguard will remove phosphates and silicates preferentially to anything else that may be in your tank, as it has a specific affinity for these two. Since you do have phosphate in both your tank water and source water, there is no need to worry about carbonates being taken out, or pH fluctuations as a result of using this media. Using Marine Buffer(a very stable buffer) will help keep your pH where it should be, as well.
For a 20 gallon tank, you should probably only be using about 1/4 cup of PhosGuard. There is a question in our FAQ that addresses the quantity issues surrounding PhosGuard:
PhosGuard is best used in smaller quantities and changed frequently. In an environment with an excessively high phosphate reading, PhosGuard will exhaust rapidly (4 – 5 days). Place the product in a high flow rate area and test your phosphate levels, changing as needed. When you see that your level is below 0.2 ppm (mg/L) leave that portion of PhosGuard in the filter. PhosGuard may yellow somewhat as it picks up organics, but that is not an indicator of exhaustion for phosphate removal. Continue to test weekly for phosphate until you see it begin to rise again. You will then know that it is time to change the PhosGuard. If you have low levels of phosphate the PhosGuard will absorb the silicate in your system. It’s inert and safe to leave in your filter.