Why did I get(have Green Water)?

Vortex it...

... And then there's the diatom filter...
I'm no plant expert, but how much, and how long do you have the lights on?

You were fertilizing a brand-new tank, where the plants haven't settled in yet. Ten days into the new set up, the water started to turn green.

Did you continue fertilizing?

What are your nitrate levels? Any new fish additions?

GW isn't unheard of in new tanks, which you have. As your tank matures, it'll become harder to get GW.

I did, however find that a small ammonia spike is enough to start GW going. Once it gets going, its hard to get rid of it.

1.) Do a 50% water change.
2.) Raise spray bars, add airstones, etc. to increase surface aggitation.
3.) Clean out the mechanical portions of your filters and shut off your CO2.
4.) Cover your tank with heavy blankets, black trash bags, etc. Make sure no light can get into your tank.
5.) Leave covered for 2-3 days, don't feed your fish during this time... don't worry, they'll be fine.
6.) Uncover tank, do another 50% water change, dose with enough KNO3 to bring your nitrates up to 5-10 ppm (if its there already, don't bother). Does your trace, pottassium, etc. to their proper levels.
7.) Keep up with the proper CO2 levels and nutrients levels, and GW shouldn't come back.

If GW is presistant, no matter what you do, look into getting a UV sterilizer or a diatom filter. I'd leave these as a last resort though... blackouts are free, try those first.

HTH
-Richer

Funny, you have a 120gal, 3 weeks old w/350w of MH's, KH 3, and onyx sand and I have a 75 gal with 350W/MH 3 weeks old, KH 3 and onyx sand also.

If it even appears like GW do a huge water change ASAP. I mean like 75% or more. Feed the plants.
When starting a new tank I add about 1/2 the normal amount of dosing except for CO2 which is sky high 50ppm etc till I add critters and fish in there.
2nd week I up it to reg dosing but 2x a week then the 3rd it's getting the full 3x a week dosings.

3rd-4th week, The horde is added, 70 Amano shrimps.
I got a slight bit of diatom and Oedogonium on the glass this week.
Plants have taken off.

Diatoms or UV's.
There are a few sources of NH4 but if the tank was not packed with healthy plants from the start of fertilizing, often it will be somewhat unstable initially.
Get the tank up and going well, water change, cleaning resetting the nutrients and adding herbivores etc.

If you have another tank, try doing 100% water change, Add old second tank water back in say 20 gallons etc and keep add the nutrients back. The plants will get going. That will remove most all of the algae. It still might come back. You can try 100% + a 3 day blackout if the plants look decent.

Remember that GW does not hurt the plants and reducing the nutrients etc will not stop it one bit once it's there. So adding nutrients/CO2 and doign large removals of GW through water changes THEN when the plants are healthy, do the blackout but it must be complete and do it immedaitely after a large 100% water change.

This way it will greatly reduce any leftover algae and the plants will be able to take a long blackout and bounce back well afterwards. Wait a few days/week till you do another water change after the blackout is removed. Don't touch the tank till things are set.

Regards,
Tom Barr