Sounds like you need to make your partial water changes more often. If you increase these changes and it still doesn't go down,and you are using tap water, than you should test the tap water for nitrates. If you have them in your water you need to start using bottled or R/O. They do sell filter media's which help absorb nitrates, though I would try to fix it at the source first. Also, keep the number of fish to a minimum since this is a very small tank - just a couple.

Since water changes are one of the only manual methods of removing nitrate many saltwater tank owners opt to use a protein skimmer to make the job easier.

Try the link below to get a whole bunch of ideas on how to lessen it.

i would do more frequent water changes, and if you can afford it, get a bigger tank. your nitrates are a little too high (you should keep nitrates preferably at 20ppm--no higher than 40 ppm).

your tank would be way too small for most fish species.

Ghapy has the best answer here so far. If you are useing distilled or RO water you will want to make sure that you are not keeping it stored for any longer then a few days as that type of water will gather nitrates just by sitting. Even if the container is sealed so long as it is exposed to the air. I think you can avoid this kind of problem if you add Marine salt to the water as its being stored. I think just that the water is so soft that Nitrates are collected from the air easily. You might also consider storage containers or object in the tank that might be releasing Nitrates into the water.

Good luck

PK

You can add a protein skimmer and/or plumb in a refugium with a deep sand bed to lower Nitrates. Fish should be okay with Nitrates in the range of your tank's though, so that may indicate another problem all together, as most corals would be casualties of high Nitrates before fish would.

Add a protein skimmer, feed less, and change your water (about 5%) weekly instead of 25% monthly.

historically fish can handle nitrates better than inverts.can.use purified water,distilled being best.being a small tank, you can buy it @ walmart,ect. or the local grocery.a good skimmer is a asset.small tanks generally are more prone to problems like this.live sandbeds help to lower these, 2"deep or so, use aragonite.what kind of test is being used?could be inaccurate.macroalgae,in a refugium,can help good luck!

Well first off a protien skimmer does not remove nitrates...it will remove disolved organics from the water. For a small tank like that it is best to have a sump to increase the water volume so you have more dilution of waste.
Out of curiosity do you have algae issues at all? Do you know your phosphate levels?
If you want to reduce your nitrates you should add a hang on back refugium with a 4 inch sand bed and some macro algae such as chaetomorph. You want to have a seperate light on the refugium on an opposite photoperiod of the display tank. This will help keep you ph stable through the night and your corals will love you for it. You want to have very low flow through the refugium for optimal conditions for algae to grow there and not in your display. If the macroalgae is growin in the fuge well than it will suck up all the nutrients allowing it to grow which in turn will lower the levels in the water. Hope this helps and fee free to email me with any questions.