280 Organic waste recycling: technology and management
pH
Huet and Timmermans (1971) stated that the best water for fish cultivation is
that which is neutral or slightly alkaline, with a pH between 7.0 and 8.0.
Reproduction diminishes at pH value below 6.5 and growth rate becomes lower
at pH range of 4 to 6.5.
Figure 6.9 Effects of pH and ammonia nitrogen concentration (NH 3 + NH 4 +)) on the
concentration of free ammonia in water (Sawyer et al. 2003: reproduced by permission of
McGraw-Hill Book Company)
Carbon dioxide
The presence of free CO 2 may depress the affinity of fish blood for oxygen. Fish
can sense small differences in free CO 2 concentration and apparently attempt to
avoid area with high CO 2 levels. Nevertheless, 10 mg/L or more of CO 2 may be
tolerable provided DO concentration is high. Water with less than 5 mg/L of
CO 2 is preferable. The most detrimental effect of free CO 2 results during the
period of critically low DO (Boyd 1979).
NH, mg/L, as N 3pH