Your tank must be starting to cycle if your seeing nitrite levels and you need to do water change when you see start to see it showing up. It should not be to long before you start to get a reading for the nitrate which is good it means your tank is almost done cycling. It will be cycled when you get no ammonia and nitrite readings.
Keeping up with the water changes i very important now. Prime is a great conditioner and it will detox the ammonia and nitrite and nitrate but only for 48 hours but you will still get a reading for the them when you do the test.
Also you really need to get heater for your size tank I think a 10 or 25 watt would be good. Keep a check on the temp of his water it should be from 76-82 for his health and to stay active.