Help High Nitrates!

Hello. I have recently set up a tank about 3 weeks ago. I used earthworm castings for the dirt and gravel to cap it. The tank is cycled but now I am having issues with high nitrates. There is no fish in the tank just plants and some snails that came on the plants. I have also having some issues with what looks like green hair algea probably due to the high nitrates. I was wondering if maybe the earthworm castings are leaching nitrtates into the water. If so will this subside or should I tear the whole thing down. Any suggestions are appreciated.
Thanks