I have a new tank which is recently fully cycled (fish-less), since then I have stocked my tank with some neon tetras, galaxy rasboras and some guppies. Last night one of the guppies was swimming at the surface which to me arose a red flag indicating high nitrites. I did water testing and my ammonia was 0ppm, nitrites 0ppm, nitrates 5ppm (all done with API freshwater master test. Only thing I noticed that is different is a sudden (as of 3 days ago) milky cloudiness to the water. What could of kiled my fish? BTW, I had crystal clear water, added new driftwood (boiled to remove tanins for several hours) it tinted the water a light amber. Everything was fine, fish liked it. I did a 30% water change at the end of the week and the cloudiness developed overnight.