It's a difficult one. You would need a hard water fish suitable for small tanks. The hard water fish which always spring to mind are livebearers, rainbowfish, central American and Rift Lake cichlids. The two types of cichlid are usually too big for your tank, as are most species of rainbowfish and the common livebearers apart from endlers and guppies which you already have.
For small tanks, Pseudomugils are usually suggested, but you can't get those. I know one member with hard water keeps Sawbwa resplendens (common name Asian rummy nose, not related to rummy nose tetras) but I have no idea how easy they are to get hold of.
For small tanks, Pseudomugils are usually suggested, but you can't get those. I know one member with hard water keeps Sawbwa resplendens (common name Asian rummy nose, not related to rummy nose tetras) but I have no idea how easy they are to get hold of.