Sorry misread the Q. Add tufa rock or shells or whatever. It'll harden the water somewhat, and the gobies will enjoy that. They also like the little caves to hide in. But it isn't essential. BBGs live in a variety of habitats including acidic blackwater streams (in the case of some species) so they aren't picky at all. All they want is hiding places and shade. Of course, anything you add to the tank mustn't create dead spots, so arrange things so you and the filter can keep it all nice and clean.
Cheers, Neale