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cyanobacteria in high current???

Hi all,

I know the subject has been discussed over and over again. I checked the
archives up to a few years ago and nothing explains to me what is currently
happening in my tank: cyanobacteria. I have had these slimy sheets before
and at that time it was clearly NO3 limitation. As I added a bit more NO3
( zero measurement  ) they started spreading less rapidly and my plants
resumed growth. ( I was suspecting NO3 limitation as my duckweed, floating
watersprite and carpet of hemianthus started desintegrating , also Ammania
S. suffered )
Now they are back, in a different colour ( dark green ), and a different
location: places of high current! The sheets are forming on the sides of
plant leaves , but only where the current hits the leave. The opposite side
of the leave where the current leaves remains clean. The place where my
sump return pipe blasts its current is overgrown by cyano's. Also near the
entrance to the overflow pre-filter there is more current and the cyano's
grow more rapid here. changing the flow places the cyanobacteria in new
high current places
I've heard about everything about cyano's. People claim they need a calm
corner of the tank ,need a lot of light, caused by a too low N:P ratio....

My question : has anyone been faced with cyano's that clearly like current
? Is this may be a filtration problem?: Water coming out of the filter
containing something that  cyano's like ( may be atmosperic N from
denitrification, or just plainly good current of N dissolved water in a
otherwise N limited tank ).
I feel I need to add quite some KNO3 to keep the plants growing since tank
stocking level is not low: 14 congo tetra's, 6 SAE, 10 otto's, 6 J.
Floridae, 14 red phantom, 1 pair of kribensis, 1 pair of ramirezi, 6
hatchet surface fish, 10 amano schrimp.

some specs:
75 G with sump 15 G( turnover 3 X Hr)  , medium to high light ( 5 X 58 W
fluorescents on electronic ballast ) , all surfaces covered with plants ( I
guess you call this dense planting ) about 50 % fast growers . KH 6-8, CO2
injection till between  Ph 6.8 to 6.99 , fertilising with TMG 35 ml per
week , KNO3 1 tsp per 3 days, K2SO4 1 tbsp per week at water change or keep
up level with 1 tsp every 3 days . No need for magnesium or calcium
addition,  water is not so soft . tap water contains few PPM of nitrate.
filtration via overflow to sump , sponge media only .

Anybody out there who can comment as this one really fights back? I can
keep the cyano's more or less under control, they don't really overgrow but
rather spread in specific places only ruining the beaty of this one year
old tank that had only few problems so far . ( two algae outbreaks that
were back under control in 3 to 4 weeks by changing water and resetting
fertiliser levels to advised nominal values. )
any reply would be appreciated,...

suisoman Dirk