The best way I have found to combat nitrates is to do bi-weekly water changes........
What size is you tank? and how many fish do you have in it? The high nitrates could be from too many fish in too small a space...

My spouse has guppy tanks really loaded down with a common houseplant called Pothos and she can go for months and months with no nitrogen (nitrates, nitrites, ammonia) problems.

I have a 5 gallon betta tank with a Pathos growing in it. Just stuck a cutting in the tank with the leaves hanging over the edge and it rooted in no time. I don't bother testing the water but the fish seems quite happy.

The Pathos will thrive completely under water or with leaves hanging over as lycopus describes. Under good conditions it may need some trimming every few months.

An aside perhaps, it apparently won't take outside light levels so it won't thrive in shallow ponds outside in bright sunlight.