Ted , check your mail.

In looking for info on this topic for Ted one interesting point came up several times. It was noted by Todd above. Acidic waters ( like those from E. Texas through the deep south) especially those with tannin have more limited bacteria. This is because tannic acid water kills or reduces some bacteria. Some bacteria are bad for fish and thus fish health. I am sure some are good and some bad and some have no effect. The writers noted the cyclic nature of the question wrt water quality. High acid (low productivity) - add lime - get more bacteria - high densities greater risk of fish problems. They were approaching the matter from a fish health perspective , noting that adding tannins or tannic acid would kill bacteria and cure or reduce certain fish ailments. Water quality is not an easy subject.