whats causing my green water?

[mr funktastic]

i have had my 20 gallon set up for a while and had it up to high light for a while but recently i found the water to be a little green so i dropped the lighting down. i had 60 watts of t-8 lighing with a mirror inside the hood as my reflector so i am getting really good light. but then i dropped it down to 40. i am running a 2 litre co2 bottle diffused with the hagen ladder then when the bubbles escape from the top of it they are sucked up by my aqua clear. so i am getting good diffusion as well. i only dose florish trace 2 times per week. the only thing i would say is off about the tank is my nitrates sit a little low around 10 ppm. using a AP test kit. i am going to add another 2 litre bottle of co2 tomorrow to up that a bunch but i dont currently have and nitrate to dose. i am a little worried since i am going away tomorrow untill the 17th. my lights go on at 9 in the morning till 12 then they are of till 2 then back on till 11. so thats 12 hours of light per day. i dont have a high bioload and it seems the plants are using up nitrates before they are being produced. i dont understand how since there arnt many plants in there nor are they big. i just replanted everything and switched to eco complete. well i have rambled for long enough so what do you think is casue my green water? is it as simple as low nitrates? here is the tank nice and clear. i will post a pic of it green maybe tomorrow.

 

[Glenc]

Green water is a real pain and can be tough to get rid of. Upping your CO2 will help. I noticed your pic that your water level is a bit low. You may not be getting the C02 levels you think. By letting your water get that low, you are gassing off alot of your CO2 as you are getting alot more surface aggitation. You would be surprised how much CO2 you are losing. Top up your tank first, then you need to test P04 levels as well to see where those levels are compared to your N03. If you don't have the test kits, you will need to get them to have a healthy high light tank. Try for a minimum of 10ppm NO3, 1ppm PO4, 10ppm K. Those levels can fluctuate a bit, just try to keep them in that range. Maintaining those levels in my tanks has kept my water clear for quite a while now. Green water in my tank has usually been caused by fert imbalance combined with to much light for to long. Good luck.