Here's some data for you...not sure if it'll help.
From Cloudy water in the aquarium. What causes cloudy aquarium water? - Algone.com
with a promotion of their product included of course...
"Bacteria need oxygen
. A few grams of bacteria consume about the same amount as an adult human, again posing a threat of de-oxygenation in the aquarium. Immediate action is required if the problem is severe, or persists.
Once again the focus should be on nutrient control
. Eliminating excess nutrients will starve the unneeded bacteria colonies resulting in a clearing of the water. A UV-Filter may be somewhat effective, as it kills bacteria. Nevertheless, the problem of high nutrients will not be fixed this way. Therefore the nutrients have to be lowered through water changes and the use of products such as Algone
Maybe the oxygen consumption explains the lethargy?
From Cloudy fresh water aquarium? - Yahoo! Answers
"Put some new activated charcoal in your filter and do a 25-30% water change. The new food is probably the culprit."