Changed the water in the jar

I try to change the water in the tank every two weeks or so. Today was the day, so I also changed the water in the baby fish "tank".

The tank really is just a clear 2 liter Tupperware jar. You can see all those little fishies swimming around in there. They are still less than a centimeter in length.

Scooping the fish out carefully with a little cup, I found that roughly half of them were dead. Was it the cold? Was it the water quality? Was it sickness? Or was it just the way Nature works?

Anyway, the water is clean again and the healthy fish are swimming all around and an occasional dead one floats along the bottom of the jar. There must be 20-30 healthy ones now. That's a cut of around 70% from the first time I culled the fries.