Ye hear of this type of thing happening quite a lot with seemingly no reason,if ye didnt find nothing with the tests, maybe you shoulld smell the water nn see if it smells different lol,
ive heard loads of times this!! and something is nearly always in the water (tank that shouldnt be there.... Common ones are dead fish that has popped out even though they have searched the tank, Likely suspects are KIDS messing about, a mate had his kid pour nail varnish remover into his tank.. hope it works out ok....