What is the best way to lower phosphate and nitrate level?

[Oilers]

I am setting up a small tank (28G nanocube) for my little boy. I used some of the live rocks from my DT and filled half the tank with the water from the DT and made up the rest of the water. It's been running for about a week under metal halide light for about 12h a day. I tested the water today and found the level of phosphate and nitrate to be a bit high. Phosphate is around 2 and nitrate is around 5. What can I do or add to the tank to lower those levels down to zero? Also, there are some brown algae on top of the sand bed. What should I do beside WC which I am planning to do one tomorrow? Is the algae forming because of too much light? I have no sump but do have a skimmer running. Thanks for your input.