My tank has been cycled for about 3 weeks and I am suddenly having a nitrite spike. I added a new dwarf algae eater two days ago and yesterday when I tested my water it showed levels near 3.0 so I did a 20% water change. Today when I got home I tested the water again and it was now up in the 5.0 range. I did a 40% water change immediately but I am afraid I am about to start loosing some fish. What could be causing this? Any suggestions on what to do?