Hi there,
You can use baking soda to bring up the PH of the water. They usually prefer about 8, but how much you use depends on what the PH is at the time you add. This will not increase your hardness however which is a requirement for tangs.You can use shellgritas a substrate and this will come pretty close to fixing both requirements, but depending on what you want to keep as to what substrate you should use. Featherfins or other similar types do best with sand whereas my tropheus do just great with shellgrit.
Hope this is of some help for you
Anita