Algae apeared in hi-tech tank - reason?

Hello. I have 17 litres tank:

1,2 watt per litre light 10 hours
CO2 - from CO2 tank 24\7 diffused via small pump impeller to micro bubble condition.
External filtration
PH - 5,9-6,0
KH-2 GH 4
Tank is densely planted with HC, R.spGreen. L.arquata, R.fluitans. E.parvula еtс.
Daily dosing NPK +trace+Fe (all DIY)
NPK dosing (weekly concentration) - No3 - 18ppm PO4 1,2 K-26,4.
Fe 0,1 per day
Water change - 70% weekly.
After change I add 20ppm of K, 0,2 0f Fe and double day dose of NPK.
Everything was fine since start in Jan09.
But at last week on second day after WC I found small green hairs (probably Oedogonium or fuzz algae) on arcuata leaves, then on the next day on the H.microntemoides and on E.tennelis rubra. I start nervous cause everything was stable. I check No3 level with Tetra test and find 3-5 ppm max (but I add such amount dayly with double dose after wc). Fe level was also checked - 0,5ppm.
So on next day i change 50% of water and add 6ppm of no3 ant night and 6ppm at today's morning. At evening I check level - aprox 7 ppm - so i decide to add another 10 ppm of no3 (i have separate solution) to give plant no limitation in no3. Did I made correct?

But the main question for me is - what the reason of the algae activation?

My assumption:

1. Im check ph level - and was surprised - may be because of some problems with my co2 system - but ph was 5,56 (its in evening time) (instead of 5,9-6,0). So may be this cause CO2 increasing which stimulate NO3 consumption, its drop to low level and algae activate? - but to do this plants must stop its grows.? isnt it? Or even low concentration of NO3 is enough?
2. May be HC start grows faster and start eat all nutrition which cause level drop.
3?? dont know.. seating here and nervous - cause it tank for exhibition and
not that presaged trouble.

So I dont want to add some chemistry and do not want to make blackout or something like this, cause it cant affect plant its stop growing and......oh my god.
Please any advise and assumption will be very helpful

thanks