It's been 3 weeks. We did a fish less cycling and when it was done we put 3 tetras and after 4 days they all started dying. Since then the reading for ammonia keeps showing .50ppm and now Nitrite .25ppm and Nitrate 5.0ppm. No fish after they all died and I'm not using any ammonia, also i bought Salifert to test ammonia its showing 0 but API it keeps saying .50ppm. Thank you guys so much.