I have a 40L with caribe complete substrate. It is planted well and running on a marine penguin filter. My fresh water ammonia tests are very high. Nitrate and nitrites test 0ppm. Have tried 50% water changes which lowers the ammonia but within a day or two it climbs right back up off the scale. Fish seem fine water is clear. any ideas as to what might be happening here. Tap water tests 0ppm ammonia.