Originally Posted by AbbeysDad
Okay, but ask yourself...if you have tore down and set up a clean aquarium, there is no bacteria to bloom...that only comes with decaying organic matter....right?
Not quite correct, please allow me to explain from my understanding.
Bacterial blooms are most common in new tanks. Established tanks rarely have them, except when the balance is disturbed. There are two types of bacteria at work in aquaria. Autotrophic bacteria use inorganic matter to synthesize its own food, and nitrifying bacteria are autotrophic (there are others too). Heterotrophic bacteria cannot synthesize its own food so it needs organic material such as waste, dead bacteria, fish and plant matter, etc.
Heterotrophs appear sooner and faster. They build many of the biofilms that all bacteria use to adhere to surfaces, and they reproduce much faster, around 20 minutes, compared to hours for the autotrophs. So if heterotrophs cause the bacterial bloom in a new tank, and yet there is very little if any organic waste yet, how? Well, when water is dechlorinated, it can suddenly support bacteria, and the "organic waste" in the water itself feeds the heterotroph bacteria and it very rapidly reproduces and clouds the tank milky white. This will occur in fishless cycling with just ammonia. It is usually less likely, or will be minimal by comparison, with live plants because they assimilate nutrients.
Unlike autotrophs which need oxygen, heterotrophs can switch between aerobic and anaerobic depending upon the environment. This is why they can kill so many nitrifying bacteria in filters when the filter is allowed to get clogged. When heterotrophs bloom in the water they switch to being aerobic and consume vast amounts of oxygen. This is the real danger of a bacteria bloom, as it can starve the fish of oxygen (if fish are present).
When the established tank's balance is thrown out, such as by overfeeding, excessive decaying plant and animal matter, excess waste from overcrowding, etc., the heterotrophs quickly reproduce by feeding on this organic matter. This produces ammonia as a by-product, and the sudden surge in ammonia overtakes the nitrifying bacteria that need time to "catch up." Live plants again help here, as they can assimilate and/or take up considerable quantities of ammonia faster. But the ammonia rise does not cause the bloom, it is the opposite.
Hope that makes sense.