There is rarely such a thing as too many water changes. If you want to do 3 a week it won't hurt anything.
As far as earlier when you asked about bottom dwellers several species of syno catfish do well in malawi set ups. They don't mind the higher Ph and can handle the aggression. My favorite is the Synodontis Petricola. They are really interesting little catfish that like groups and will come out and swim about. They only grow to about 3.5" - 4". Really cute addition to a mbuna tank
As far as earlier when you asked about bottom dwellers several species of syno catfish do well in malawi set ups. They don't mind the higher Ph and can handle the aggression. My favorite is the Synodontis Petricola. They are really interesting little catfish that like groups and will come out and swim about. They only grow to about 3.5" - 4". Really cute addition to a mbuna tank