Is KH the same out of the tap, or is it being depleted once its in the tank?
If pH is neutral (7.0) out of the tap, but is 6.0 in your tank, you either have some pretty significant bioacidification going on, or you water may be treated with the lime-soda method by your water utility. (See
http://www.skepticalaquarist.com/softening.html for an explanation.)
To me, the most likely scenario is that your water has little KH coming out of the tap, and the 25% water changes aren't enough to keep ahead of the acid produced as ammonia/nitrite are oxidized, causing pH to drop.
If this is what's going on, adding buffers will do the trick. Crushed coral, either as a substrate or in the filter may be enough, but it might not dissolve fast enough to keep KH and pH up as high as you'd like for africans.
I use epsom salts to bring up GH and a combination of baking soda and Aragamilk (a liquid carbonate solution) to bring up pH. You most likely will have to experiment with proportions to reach the GH, KH and pH you desire, but my water is similar to yours and I mix one bag of epsom salts (I forget the weight, but it's about the same size as a bag of flour) and two or three boxes of baking soda together. Once mixed, I add about a tablespoon per ten gallons of water. I also add about a tablespoon of Aragamilk per 40 gallons of new water. This gives me water that is very high in GH (around 400ppm) and KH (about 260 ppm) and has a pH of about 8.0.
If you try this route, be sure to experiment with adding these to water that's been sitting at room temperature for several hours, to allow any dissolved gases (like CO2) that might affect pH to offgas.
Good luck and have fun!
Jim
