The nitrifying bacteria are very efficient at surviving and thriving on very low levels of ammonia and nitrite. They do well at all household aquarium temperatures, heated and unheated. No need to set the thermostat for your bacteria, choose a temp that is best for your fish.
"Optimal temperature for Nitrosomonas growth is 77 to 86 degrees, which fortunately coincides with the temperatures we are likely to keep our tropical fish. For aquarists keeping cold water tanks, Nitrosomonas growth is reduced 50% at 64 degrees, 75% at 48 degrees, inactivation at 39 degrees, and the bacteria die at 32 degrees. They also die at the high temperature of 120 degrees. " from
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