You could try some floating plants like Hornwort or Duckweed but the best option is RO water. You could use half RO and half tap as that would get you to around 20ppm which is still to high.
I say 20ppm is the highest I'd ever let one of my tanks get to, high levels of nitrates can have all sorts of effects of long term exposure leading to death. Some examples include:
It decreases the o2 levels in the aquarium.
It decreases the ablility for reproduction.
It decreases the antibody production which help fight disease.
It causes long term damage to spleen, liver, and kidneys.
It causes lower red blood cells which regulates health such as clotting.
It leads to damage of eyes causing blindness.
It leads to death, the worst thing on the list!
I could go on and on but I'll stop there since I'm sure most people get the point. These things start to effect fish at levels of 60ppm, at levels in excess of 200ppm can lead to death within a few weeks.
Where are you located? The EPA has a law that nitrates can't be higher than 10ppm, if mine where that high I'd be more concerned about my family than my fish.
Basic Information about Nitrate in Drinking Water | Basic Information about Regulated Drinking Water Contaminants | US EPA