If you want to keep the African cichlids;
First of all, you'll need a separate tank to move them into, filled with the same water as they have now (and, don't forget, you'll need to move some cycled filter medium to the new tank's filter when you move the fish, or you will have ammonia spikes). It need to be decorated with a lot of rocky caves. Don't bother with plants; these fish don't live with plants in the wild, and will just rip them up and make a mess, IME.
Then you can make up your hard water, by dissolving the minerals (they come as a dry mix that looks a bit like salt) in some warm water until it's the hardness you want. Then use that water to do your water changes; you'll want to do a water change every couple of days, as you'll need to get the hardness up quite quickly, but not so quickly that you shock the fish.
Once you've acclimatised your current fish, you will need to add more cichlids; because these fish are so aggressive, you must keep them in an overstocked tank, or the stronger fish will be able to pick on and bully (and very probably kill) the weaker ones.
Because you need to keep Rift valley set ups overstocked, it is essential that you keep a good maintenance regimen; a big filter, preferably an external, and lots of water changes, using your hard water mix. I used to change about 75% weekly on mine.