dannios dannio breathing

110 litre Ammonia 0 Nitrite 0 Nitrate 15 Ph 7.2 Temp 26 Water 50/50 RO and tap changing 30% weekly Good water movement and filtration show up as 8/10 on oxygen test kit. Air stone in the tank along with the filter bubbler. One of my 4 dannios has started breathing very rapidly, around 5...