You also need an ammonia test....
Nitrate test will give a false positive when there is nitrites in the water. The nitrate test converts nitrates back to nitrites so when one is having readable nitrites, there will also be a nitrate issues. Same applies for some other ions so nitrate is a bad way to evaluate water quality.
My point is, your tank isn't cycled. I'd step up the water changes big time and as others have mentioned, make sure to dechlorinate the water. You've got a tough fish in there....
Long term, you need more filtration as well. That and water changes will keep your tank spotless.
