Yes, 13 dGH (the German number) is in the fairly hard range. This article may explain how all this relates: http://www.tropicalfishkeeping.com/f...quarium-73276/
Our fish profiles give the preferred ranges for each species, you should take a look at those too.
Without any changes, livebearers will fare well. And there are some other fish among the cyprinids, athernids and catfish. Plus the African rift lake cichlids, though a 25g tank is not sufficient for most of these. Several of the characins can adapt.
Extreme soft water fish like cardinals will not have an easy time in hard water. Adjusting the water parameters is possible, not via chemicals but by dilution with pure water (RO, distilled or rainwater works). This is easy enough initially, but obviously weekly water changes have to take this into account too.