The hygrophila in my aquarium seems to be developing root rot. The planted
roots turn brown, then the stim slowly turns brown and decays. Floating,
they are perfectly healthy. The plants are obviously not to blame. They
can be planted in my other 20 gallon with no problems. However, the other
20 gallon has been setup for almost 8 years. This "problem aquarium" is
only three weeks old and has just finished its cycle. Could this be a
factor? Anyway, my water specs are below...

PH: 6.9 (Via Aquarium Pharmaceuticals PH Down [non-phosphate]), GH: 7
degrees, Temperature 76F, Ammonia: 0, Nitrite: 0 , Nitrate: 10-12ppm, CO2:
..8ppm.

Water additives: Aquarium Pharmaceuticals - Tap Water Conditioner, Stress
Coat, Stress Zyme, PH Down, Leaf Zone, Root Tabs (8).

Substrata: Natural Gravel 4-6mm, three inches deep, implanted with Root
Tabs (Iron, Potassium, Magnesium)
Filtration: TetraTec PF150 using activated carbon - 150GPH, DIY CO2 Injector