09-08-2011, 11:39 PM
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"If a UGF has been in operation for any length of time, solid organic waste may be in abundance in the gravel and under the plates. While the filter is running properly, these degrade aerobically into compounds like nitrates and phosphates which may be algae stimulants and fish growth inhibitors, but at least are not generally toxic. If water flow is halted, oxygen becomes unavailable in the gravel, and these same waste products break down anaerobically into much more noxious compounds like hydrogen sulfide and methane."
This is true as far as end result but what is missing is the reason. Organics do not break down into anything without bacteria. And it is the sudden and considerable increase in the bacteria that causes the hydrogen sulfide and such. Citing from my bacteria article, Heterotrophic bacteria cannot synthesize their own food so they need organic material such as fish waste, dead bacteria, fish and plant matter, etc., and while some are aerobic, many are facultative anaerobes, meaning that they can survive in either the presence or absence of free oxygen. Anaerobes are organisms that do not require free oxygen for growth. This has significant consequences in aquaria. These heterotrophic bacteria, of which there are several species, utilize nitrate by consuming the oxygen within nitrate and releasing nitrogen gas. They do not require free oxygen in the water so they are facultative anaerobes, and generally occur in what we term “dead spots,” which occur when water movement is stopped and thus no oxygen is available. These species of heterotrophic bacteria break down dead organic matter like fish waste, dead fish or plant matter, uneaten fish food, dead bacteria, etc. Some are aerobic, but many species are facultative anaerobes, able to live with or without oxygen. Like all bacteria, they colonize surfaces, and these are most prevalent in the substrate and the filter media. Many species can survive complete drying, allowing them to remain potent even when filter media that has been previously used is completely dry. These bacteria have only one requirement to appear and live: organics. They compete with autotrophic bacteria for both oxygen and surface area; studies show that even in relatively clean environments, they occupy more than 50% of the available surface area. And given that they can reproduce within 15-60 minutes—compare this to the 12-32 hours required by nitrifying bacteria—you can see how easily these heterotrophic bacteria can overwhelm the system. In a filter, if sludge is allowed to increase, heterotrophic bacteria will multiply so fast they actually smother and kill the autotrophic nitrifying bacteria.