Green water! What to do?

Hi!

I have 55g aboout a month and a half old, moderately planted tank. I
also have 5 ottos in it. It has 100% fluorite substrate, about 2W/gal
light and dyi CO2.
The plants were growing pretty well at the beginning but slown down in
growth and started getting hair and blue algae.

I have tested for amonia and nitrates (aquarium pharmaceuticals test
kits) but couldn't see any.
I figured that the ottos did not get my plants enought nitrogen
nutrients so I decided to add Seachem Flourish Nitrogen (I think it's
potassium nitrate). I didn't want to overdo it so I put one dose the
first day. It seemed to help because my amazon started putting out new
leaves. I tested amonia and nitrates again but didn't see any. So I
added more Flourish Nitrogen. And then it all started. The water
started looking cloudy. My tests still don't show any amonia or
nitrate.

Is it possible that the Flourish Nitrogen puts in Nitrogen in a form
that the tests cannot detect but the algae can utilize?

How do I clear out the water besides doing water changes? I usually do
about 25% weekly.

Also, the leaves on my amazons and anubias started looking wavy and my
chain swords' new leaves look pale. I must be lacking some nutrient in
the tank. Any recommendation on what I should add? I think I read that
iron deficiency can cause yellowish/pale color. Iron shouldn't be a
problem since I have a fluorite gravel. But maybe it's the lack of
magnesium stopping the plants from using the iron.

Any suggestions? Product recommendations?

Thank you!