A bacterial bloom can occur in a matter of hours, under the right circumstances. The heterotrophic bacteria responsible for bacterial blooms cannot synthesize its own food like nitrifying and denitrying bacteria (good bacteria responsible for converting ammonia into nitrate). It is completely dependent on complex organic substances for its nutrition. These bacteria break down uneaten fish food, fish waste and dead plant matter like nori, converting it into ammonia. They also reproduce far quicker than nitrifying bacteria, in as little as 15-20 mins. This is much faster than the reproduction rate of ‘good’ bacteria that can take up to 24 hours.
In a new setup, when the aquarist is reliant on nitrifying and denitrifying bacteria establishing the nitrogen cycle, however the bacteria responsible for bacterial blooms is reproducing at a much faster rate. It is utilising the organic waste in the sand and the potential die off from live rock.
In an established aquarium, the heterotrophic bacteria will utilise organic waste mainly in the sand, and if the waste is excessive, they can multiply rapidly causing a bloom.