You must keep at least 20% of old water. suddent change of 100% are not really good to the sick fish. your fish might feel stress about it.

ari, i used this method to cure my malawi before. Aquarium salt is the best medic to cure any sickness or killed any disease. As Jimmy said, the ph is quite low. Cichlids will only surviving or growing well in higher PH. This is the basic knowledge of keep cichlids.