raising gh

I'm keeping livebearers: platy, molly, guppies. The platy and guppy do ok, but the molly's always have issues. I lose fish every now and again to common livebearer diseases. My tank parameters have always been good. I use an Api kit. Ammonia: 0, Nitrite: 0, Nitrate: 5-10, Ph 7.8, temp 78. Water...