blue-green algae


Blue-green bacteria has started to appear in my tank a couple of weeks
ago. It started below the gravel surface beside the glass and moving
upward. They only appeared at certain spots in my tank. 

Q:Should I be alarmed by this? 

I've read the FAQ and articles in the Krib but there is no precise 
rule mentioned to prevent them. There are hints here and there. 

Q:I would like to know what level of measurable water quality to keep 
them in check and prevent them from taking over my tanks. 

My data:

50gal with 80W lighting and Fluval 303 filter. Lighting 12 hours daily
+ influx of daylight up to 10am.
Water change 20% weekly, add Tetra FloraPride, Natrium Bicarbonate
Tetra Crypto - 2 tablets forthnightly
pH about 7.0
Ammonia and Nitrite = 0
Nitrate <12.5ppm (resolution for Tetra test kit)

Major plants are hygrophilla difformis, various Echinodorus sp., 
aglaonema, limnophilla sp., ludwigia repens etc.

Fishes are 4 zebra danios, 2 black mollies, 2 corycat, 1 pleco,
1 flying fox, 2 bumblebee gobies and just yesterday added 3 pencil

Q:I would like to add 8 cardinal tetras, 4 corycat and 3 glass
cat in the next one month. Would this aggravate the situation?
I would appreciate any response to this mail. Thank you in advance.