The 0.5 nitrite level is twice what would be well tolerated. Both ammonia and nitrite should be less than 0.25 ppm always. During a fish-in cycle like you are doing that is accomplished by big enough and often enough water changes. If you are at 0.5 on either one, a 50% water change is needed just to get in spec. Don't be surprised if you need to do a large water change again tomorrow. Until you get through this part of setting up a proper cycle, frequent large water changes will be quite common.