I have been cycling my tank for around 4 weeks now and have been doing a fish in cycle. During the whole cycle my tank has had 0.25 ammonia and 0 nitrites and nitrates. I tested it yesterday and my nitrates were 5ppm. I re tested today as one of my guppies had given birth last night and they were somewhere between 0 and 5 today. What is causing them to drop? Also, why have I never had any readings of nitrites?