OK, so the taqnk size is pinned down, and the Water is moderateely soft/hard (sort of in the middle with GH 9 dGH, and you're right, this will remain basically the same unless you do things to target it). So look at fish preferring softer rather than harder water. Do not consider any livebearers, they need it harder. But most if not all of the commonly available fish from SA and SE Asia will be fine, including those already in the tank.
Before we can suggest additional fish though, we need to know what you intend doing with the danios.