cryptocoryne usteriana

I have two Cryptocoryne usteriana in a 75g planted in Flourite Black Sand doing poorly. Very slow to send up new leaves and they eventually partially melt. Tap H20 is ph7.6-7.7 (rested 24hrs); GH 7-8; KH 4-5 - all according to API test kits. I live in the Cincinnati area which means a limestone...