Nitrate limitation -> cynobacteria?

Hi knowledgeable people,

since some weeks I have a slight cynobacteria problem. I have the impression
that it got worse since I use mostly RO water (only 5% tap water 
according to Dupla's recommendations) + Dupla KH/GH Bildner for water changes.

Yesterday, I bought a new nitrate testkit (JBL) and found out that my tank
water contains 0 mg/l nitrates. I have the impression that because of
the lack of nitrates phosphates are not used up by the plants 
-> cynobacteria. Phosphate measurement showed 0.1 mg/l.

For the water change yesterday I used 2:1 RO water to tap water to add some
Problem is that my tap water contains 11 mg/l nitrates but also 0.2 mg/l
phosphates so I am still introducing phosphates into the tank, maybe 
more than can be used up even with the additional nitrates. That's why I
started to use mostly RO water. 


Are my assumptions about nitrate limitation -> phosphate build-up 
-> cynobacteria correct?

The target I saw here on the list is to keep the tank at 5 mg/l nitrates
to keep plant growth at best. True?

What is the relation of phospates/nitrates uptake by aquatic plants?

Because of the phospate I don't really like to use tap water. Is there an
alternative? What can I put into the water to get nitrates to a correct
level? Do you also have a chemical formula or description in case they 
don't know your recommended substance here in Germany?

I use the complete Dupla system and no PMDD (just don't have the time). So
I don't like to switch fertilizers but simply add something to enhance
conditions. There shouldn't be other limiting factors. Light is 2 x 125 watt
HQL for a 60 gallon tank. At a KH/GH of 4-5, I use PH-controlled CO2 injection.
According to the tables, PH 6.7-6.8 should be optimal for the CO2 level.
I also have undergravel heating that should help to prevent cynobacteria
as some sources state.

What I don't really understand: I thought that plants first use ammonia and
then nitrates. I should always have a detectable level of ammonia in the tank
(discus eat a lot) but this is not critical at a PH of 6.7-6.8. 
Do I need both ammonia and nitrates for plant growth?

What do you think? Thanks for your thoughts.

Andi from Munich, Germany (where spring seems to come with 10 Celsius + sun)