You can grow plants at kh13. I live in florida, and springs are full of plants in what is essentially liquid rock, as far as kh goes. Make sure you get your co2 in the 20-30 range, if you are not supplying it, supply it, it will make a huge difference. Also, as Ken said, how much light do you have? It really is a matter of balance. To paraphrase Tom Barr, give the plants what they need, the rest will fall into place. Also, you do need some 'clean-up' crew, if you don't. Ottoes, sae, shrimp.