I have been fishless cycling my 60litre tank (using ammonia) for just over 4 weeks . I had the ammonia spike and then nitrite spike then both dropped to zero while nitrates went up . Apparently all i had to do next was a large water change then im ready for fish ! . i carried out a 90% water change making sure i dechlorinated the water using tetra aquasafe before adding to tank . After which i tested the water again and found ammonia 0 , nitrite 30ppm(at least) , nitrate 20. Does anyone have any idea what might have gone wrong ?