Black and curly leaves of Hygrophila polysperma

["Daniel Larsson" <defdac at hotmail_com>]

Whats up with my polysperma? New leaves are small, stubby, twisted, curly
and black:
http://defdac.tripod.com/sitebuildercontent/sitebuilderpictures/h_poly.jpg

Tank specs:
300 litres, 1-5 ppm nitrate, 1-3 KH, 6 GH, 0-1 ppm PO4
4x36 watt grolux
1,5 bubble/sec CO2
Every week after 25% waterchange I add:
1 tablespoon of K2SO4
1 tablespoon of MgSO4
1 teaspoon KNO3
30 ml Tropica Master Grow
In the substrate I use "Pokon-sticks" for additional N,P,K (12-6-8):
http://www.pokonchrysalusa.com/prods/151-177.htm