What afro said use a reef base substrate like crushed coral, aragonite sand maybe a few pieces of Texas Holey Rock, this will aid in the hardness staying in the range you need it to also your fish will thank you as well, well if you go the Malawi/Tanganyika route that is..
A good substrate is Caribsea aragonite substrates and they have a lot of varieties to choose from including crushed coral down to suger size sand with different colors..
I used a bag of crushed coral, a bag of pool filter sand, and some black sand I bought at the LFS. Coral for the structure. My pH stays at 8.4 in the African tank, and comes out of the tap at 8.2.