I set up my 30 gallon, added an old filter cartridge, and let it run through a cycle. Then, about 3 weeks ago, I added my golden wonder killifish to the tank. Everything was fine until I tested my levels a few days ago and both the nitrites and nitrates were way high up there. The nitrates were actually so high that the color on the test strip was roughly 2 times darker than the highest color range shown. Other levels showed a ph of 7.8, fairly hard water, and no ammonia. Since I was wanting to introduce cichlids into the tank, I bought Amquel and put in the recommended amount, despite that my killifish didn't seem to be having any issues with the high nitrite/nitrate levels. Within hours, he was dead. It's been 2 days now, and the levels are still the same. I'm at a loss as to how to proceed from here, and I'm very unhappy that I lost my beloved Killer. Any suggestions? I'm afraid that if I do a massive water change, it will kick start a whole new cycle.