Guideline for Interpretation of Dissolved Oxygen Readings: For mg/L measurements:

0-2 mg/L: not enough oxygen to support life
2-4 mg/L: only a few kinds of fish and insects can survive
4-7 mg/L: acceptable for warm water fish
7-11 mg/L: very good for most stream fish including cold water fish

About the image transformation: The bitmap "melts" and copies itself downward. DO readings in the 1 or 2 range, or low amounts of oxygen in the water cause the image to degenerate more rapidly.

Source: http://www.gvsu.edu/wri/education/manual/oxygen.htm