I'm guess that's GH not KH, but KH is probably pretty close. A reading of 300 ppm (which translates to 17 dGH) puts your water in the hard range. Livebearers are the most common fish that fit into this range, but don't think you are limited to the regular guppies and platies. There are many more livebearers out there than them; you just have to research and find ones you like. Rainbow fish are another option. Celebes Rainbowfish (Marosatherina ladigesi
) are small, beautiful, and work well with your tap water. Some of the cories can go up that high, too.