The real surprise is the red line of Fig. 11; it documents the substantial increase in bacteria/mL for a control water sample that was not treated with EtOH. We simply removed about ½ gallon of water from the tub at the beginning of the experiment, and placed it in an open Tupperware container under fluorescent room lighting. This control portion of tank water was sampled for bacteria content daily just like the EtOH-treated larger tub water sample. Remarkably enough, the bacteria population in this control sample experienced a slower but still significant rise, increasing to a maximum of ~ 27x the original level after 3 days. What is fueling this bacterial growth? Since we have already determined that the bacteria population in this aquarium water sample is carbon limited, it seems reasonable to hypothesize that the Tupperware vessel itself is providing a metabolizable source of carbon!
These observations prompted a more direct examination of the actual Rubbermaid tub's capacity to contribute to bacterial growth - maybe the Tupperware vessel was unusually contaminated with organics. In this experiment, we removed a 30-gallon water sample from the KSF aquarium and transferred it to the uncovered Rubbermaid tub, as described previously. Water circulation was provided by powerheads but otherwise the system remained undisturbed. Water samples were withdrawn daily for 5 days, and the bacteria/mL counts are displayed in Fig. 12. Clearly, the tub itself (or organic material coating it) is providing a suitable carbon source for bacterial growth. The bacterial population rises some 26x over the initial value over 2.3 days, before declining slightly; a rationale for the burst in bacteria population at day 5 cannot be offered at present.