First off, welcome to Tropical Fish Keeping forum.
I have near-zero GH and KH in my tap water, and the pH is 7.0 because the water folks add soda ash to raise the pH (this has no effect on GH). My tap water prior to 2001 was like yours, down below 5 in all likelihood.
Hardening the water and raising pH is much simpler than having to go the other way. There are many members here who would give almost anything to have your or my tap water.
Soft water fish from South America, SE Asia and Central Africa would be right at home.
But, if you really want the opposite, the easiest way is by using a substrate that is calcareous (primarily calcium and magnesium based). CarribSea make one of crushed coral and aragonite, available in a fine gravel and a sand, and there are certainly other manufacturers with similar products. Since rift lake cichlids should have a sand substrate, using their sand product as the substrate solves your problem. It will add calcium and magnesium to raise the hardness, and the pH will naturally rise too, well over pH 8 with a full substrate. Which is what you want for these cichlids. There is a specific rift lake cichlids sand product, much the same sort of thing, but likely more expensive.
Given the fish, this is the easiest and least expensive method. You can use the buffers and such, but that will be much more costly. The calcareous sand is exactly what you want for much less money long-term.
With respect to water changes, once the tank is established, the natural buffering will partly over-ride new water, but I wold keep the volume per change lower than otherwise so as not to push the envelope, so to speak. This will sort itself out once the tank is running, and as every aquarium is rather unique due to the biology, you may have to experiment a bit.
If you want to read a bout the relation between hardness and pH, here's a fairly straight-forward article I prepared: http://www.tropicalfishkeeping.com/f...quarium-73276/
There are also some members with vast experience in these fish, and they will likely comment when they see this thread.