Algae fighting

Hi folks, 

I wonder if anybody can give me a tip on how to fight algae.
I read a lot on the net already but nothing seems to work.

I have a 100 liter (should be close to 30 gallons) tank, fresh water.
12 neon tetras are in the tank, 2 corys, 3 ancistrus and a male betta. 
Total fish length (when grown up) is about 100cm, as we say here that you 
should not put more than 1cm fish per liter water.

Water is ph-controlled with a ph-meter and CO2-injection, ph is always 6.95.
CO2 is about 20 mg/l. My hobby is to test the water parameters, here we go:
nitrite <0.1 mg/l
ammonia 0
nitrate 15-20 mg/l
O2 8 mg/l (temperature is 26-27C, should be close to 80 Fahrenheit)
KH 5
GH 9

Some green thread algae are starting to grow (the type that looks like a
short green fur) and some black brush algae (short and long), also green spot
algae. The thread algae grow more in the regions closer to the light.
My understanding was always that that algae come from high nitrate and
phospate values, so I bought a phospate test kit yesterday, it's only
0.1 mg/l. Maybe the nitrate is too high? I use a lot of reverse osmosis
water and only some tab water for my water changes (30 liter every second week) 
because the tab water already contains 15 mg/l nitrate and I feed as little 
as possible. 
Is the nitrate the problem? Can I use a filter or something to discard
the nitrate?

The plants are hygrophilia, anubia, cryptocoryne, alternanthera, 
the tank is planted rather heavily and they are growing quite fast.

Another point is the light. My store recommended that I start with 8 hours
per day and increase half an hour per week. The tank is now 3 months old and
this week I reached 11 hours but I have the impression that the algae are
getting worse every week with more light. The store recommended that I 
reduce my light to 8 hours again, I reduced to 9 hours yesterday and also
bought a siamese algae eater, let's see if this helps.
I have two 15 watt neon tubes with reflectors, one reddish and one bluish.

And I use some tablet fertilizer in the gravel close to the roots and
Duplaplant 24 (one drop every day, recommended is two every day).

What's my mistake? Can you pls. provide some advice?

Thanks a lot, Andi from Munich, Germany.