I have a 6 gallon Marineland tank that is stocked with two neon tetras and a serpae tetra for the past two weeks. Also a blue snail. The water keeps testing >8ppm ammonia. I have exchanged the full tank of water over this week and used ammonia stress relief. My nitrate and nitrite levels are normal. The fish are acting normal. Also changed the filter. How do I get the ammonia level to normal?