The GH of Lake Malawi is normally high around 300 ppm. A tap water of 500 ppm should not be problematic for a rift lake tank.
GH can be measured a number of ways, so interpreting it properly is important.
Where I live in Panama (Central America, GH and KH fluctuate seasonally, and get quite high, the test strip is on the right
Just an observation, the 500 ppm GH would "not" do well for a tank of South American, or Amazonian soft water species .