Plants Not Growing And Dieing Off Slowly.

Dear All
My plants are not growing and are diying off slowly over few months. Could any body tell me whats going on?
Situation:
- Capacity 29 Gallons+-
- Light -  50 watt (6500k). On for 12 Hrs/day
- substrate - small Gravel + coarse sand mixed.
- Fertilizer=  PMDD + potassium permagnate(low dose) + local organic fert prepared for ext plants mixed with water. Dosed 30ml twice a week.
- CO2, moderate level, passed through powerhead.
- Plants - Vallisneria, Sagiterria, and some Java moss
- Temperature - 25 Degree C
- Ph 7 (Boring Water)
- Ammonia - 10ppm
- Fish - 3 tetras, 10 guppies. and I feed moderately.

For the first month it was doing fine but it started to get brown or dark spots all over the plants and all over the rocks. I learned that they are brown algae. I vaccume fish waste once a week, only partially.
Now my plants have stopped growing and everything is covered in thick brown layer. It can be removed by scrubbing has killed many plants and comes right back. Initially for few weeks plants were giving new leaves but has stopped.
I recently read that this could be possible due to low fert dose and low light or ammonia.
Please suggest.