Ah, it's 2 separate tanks, one for soft water fish, one for hard water fish.
If you have hard water, that's good for the hard water fish but it needs to be softer for soft water fish. The usual way to do this is by mixing hard tap water with pure water - distilled water, reverse osmosis water or even rain water if there is no air pollution and you can guarantee a constant supply. Mixing tap and pure 50:50 will halve the hardness.
To make soft tap water harder for hard water fish, more hardness minerals need to be added. Using crushed coral as a substrate or in a bag in the filter can raise hardness slightly. Another way is to use Rift Lake Cichlid Salts. This must be added to new water at every water change before it is added to the tank; and the exact same amount should be added to the same amount of new water every time to keep the tank levels constant.