You likely want the higher levels for your fish (I'm not as familiar with African Cichlids exact parameters).
The solids are present, and as evaporation of the liquid of the tank reduced water, just doing top offs of water continues to add more minerals to the tank as the minerals don't evaporate. Removing water can reduce levels of mineral content GH/KH.
Bottom line is that if you continuously do more top offs (unless it was RO water) than pwc you can end up with higher levels than you would want. You can always adjust to the number you want by doing more pwc (because you want more clean water and and adding less minerals (as in bottled additives) to make your water to the correct level necessary, or add RO water till it is where you want it to be.
You might already know that. [But I learned it from doing too many top offs and not enough pwc. Where I have 0 KH from the tap and moderate to low GH and after getting my new test KH/GH test I was puzzled why my KH test wouldn't work (just seemed it didn't because I didn't have any to detect) and why my GH was really high. :/ Always learning!!!] Test kits are amazing. I didn't know I would be doing "chemistry" in my living room. lol
