Calculating Turn-over Time for Aquarium Devices

Bruce Geist

March 31, 2003

Abstract:

In an aquarium, water is often passed through a device such as a filter or an ultra-violet sterilizer, and the processed water is returned to the tank. Question: how long before all or most of the water in the tank has been processed by the device? The question is a little trickier than it may first appear. Usually, the processed water is immediately pumped back into the tank, where it mixes with the unprocessed water. Input to the device then comes from the mixture of processed and unprocessed water. Any estimate for for how long it takes to process all the water should take into account this mixing of processed with unprocessed water. (Dividing tank volume by flow rate does not give an accurate estimate for how long it takes to process all the tank water, as will be shown below.) Formulas for determining ``turn-over" time, i.e., the time it takes to process most all water in the tank, are derived in this article.