I tried growing Streptocarpus years ago but I didn't have a lot of luck with them. I don't see any rotting leaves in your photo. I wonder, when you say the leaf edges are rotting ... are they getting brown and soggy or are they turning brown and brittle, crispy-like? In my experience, rotting leaves usually occur at the base of the leaf and stem and it's caused by over watering. Brittle, crispy leaf edges are usually do to a lack of humidity and if that is the case you can raise the humidity levels around your plant by sitting the pot on a tray of moist pebbles, replacing the water as it evaporates. Or, you can mist the leaves a few times a week. If the leaves are truly rotting, the solution would be to go easier on watering. If the soil is retaining too much moisture you can add extra perlite to lighten it up for better drainage.