Tank 2 years old running in very good conditions never crashed etc Regular WC every 7 to 10 days 25L (Tank 275L with sump) All water parameters are good. Fish excellent health... This is what happened: on Saturday evening cleaned the glass, skimmer and filters (normal) decided to add phosgaurd as the previous ones where yellow already. yes i did rinse the phosgaurd 1st. tank was fine sunday everything looked normal. Then monday after work i switch on the lights and notice my fish are mia! my yellow tang and one cardinal dead. my regal laying on its side breathing very rapidly. my surgeon and indian sailfing also with open mouth. they seem the strongest. my other cardinal and 3 clowns seem fine. Did an immediate 50L wc and removed phosgaurd as that was the only change I did last. regal starts to come right hours later. next day (Tuesday) lights on other cardinal dead... regal on its side but the other fish are doing ok. did another 25L wc. I did tests after all wc's all constant and fine. wednesday regal suffering still... thursday he was swimming and he is swimming now... he eats etc... Could it have been the phosgaurd that killed my fish?