Although the ocean recieves direct sunlight of all available spectrums - our little indoor tanks don't always do so good with this form of light.
The high "nitrate" should be looked at closely, and it's source determined.
If it's the biproduct of natural nitrogen cycle - your bioload - then water changes and going to a DSB may help.
If the nitrates are in your tap water - then doing more water changes with this freshwater source can actually make things much worse.
I like ruaround's suggestions.
Test for phosphates - tankwater and freshwater.
Test for nitrates - freshwater
Reduce or block this sunlight, and possibly reduce your electric lighting period as well.
If you find readings of phosphates and/or nitrates in your tap water - don't use it anymore.
Invest in an RO or RO/DI unit, or get your freshwater from another source.
I would maybe go with DANiel's approach as a last resort, but only after checking everything else out. More likely I would start over and add a DSB if you don't have one now. I've done it before - it stinks - a lot of work involved - but in the long run I was glad I did - it was worth it to me.
