The nitrites rising is a good thing right now (normally you would want 0), however it shows that bacteria are breaking down the ammonia. Keep up the 20% waterchanges to keep the fish happy and the ammonia low, stay away from the gravel though, and keep monitoring, you should see ammonia drop, nitrites climb a bit more, then start to drop off, leaving you with 0 ammonia and 0 nitrites, and only nitrates.
Once you get to there, it's the weekly waterchanges that keep the nitrates down, the other two should sit solidly on zero