The Blackmast only occur in limestone country, where the water is hard and alkaline. For example, they occur in the Katherine River near Katherine NT, but do not occur further upstream where the water is softer (upstream the water is coming from the higher Escarpment region).
We suggest keeping them at pH 7.6-8.2, general hardness 250-400ppm, and in aquarium store tanks, adding salt as well (5ppt). In the home aquarium the salt should not be needed, but they should still always be kept in hard alkaline water. When kept in soft water Blackmasts can be very difficult to keep healthy, they are also extremely sensitive to handling and transport stress when kept in soft water.