I just started my first tank about a month and a half ago. I cycled my tank and all my levels were good so I started adding fish. It's a 29 gallon and right now I have 3 Cory cats, 6 neon tetras, and I had 5 black neon tetras but I'm down to 3 now. I've been checking my ph, ammonia, nitrite and nitrate levels weekly and the other day I noticed one of my black neons was dead and I checked everything again. Everything is fine except my nitrite level. It's at about 50 ppm. I did a 50% water change and added some water conditioner and quick start (for the live bacteria) but my nitrite hadn't gone down and I've lost another black neon. What do I need to do? How do I get the nitrite down? It's been 3 days since I did all of this. I'm worried about my fish. Please help.