At the time when pale died, foam appeared on the surface of the water. I thought this was a bad sign and used a test kit for hobbyists to check the water parameters. The nitrate level was slightly increased, but not in a range that should pose a problem. Zebra fish are generally considered to be easy-care fish.

More striking than the foam was the fact that the water fern (declared as Salvinia molesta by the aquascape shop where I got them) was developing extremely well at that time, an obvious sign that something had changed in the water parameters.