impoper cycle i think(lack of good baterical in water).. i face this problem also.
i quickly change 50% of NSW and then start weekly change 10% water(NSW)
water evapourate i top up RO water (baterical added) ... water change use NSW(Iwarna)...
Recently tonnes of dead fish spotted at Lim Chu Kang, Sungei Buloh, due to lack of oxygen in waters. will this affect NSW ???