I had a lot of my fish died suddenly in my 55 gallon tank (3 cardinal, 2 firefish, 1 yellow clown goby and 1 royal gamma basslet). No visible marks on the fish except for the firefishes were swimming upside down. Water parameters are all stable (Nitrate - 0, Ammonia - 0.03, Phosphate - 0.08, salanity -33, pH - 8.2)