http://www.reefkeeping.com/issues/2005-10/rhf/
Here's the article and the whole premiss for the idea of the continuous water change. Here's an excerpt from the article about this:
Small batch water changes: These changes are similar to the large changes above, but are much smaller and are done more frequently. Daily changes of 0.25% to 2%, for example, can be used. One could also do a series of consecutive small water changes on the same day. This method ensures that organisms near the top of the aquarium are not exposed to the air, and that water parameter shifts are less sudden. These types of changes can be done in a variety of ways, such as by removing water via a skimmer and replacing it once a day, or by simply taking out an amount (such as a half gallon) and replacing it once a day (automatically or manually). While lots of smaller changes (say, 30 changes of 1% each) are slightly less efficient than one larger one (30% in a single batch), the difference is small (30 changes of 1% each exactly matches one 26% batch water change), and consequently other factors of convenience or stress on organisms may be more important.
In doing batch water changes of 2% or less, aquarists need not particularly worry about the changes in the water parameters that will result, as long as the new water is of reasonable quality. For example, a 1% change with new water at 55°F from a basement reservoir will change the aquarium temperature only from 81°F to 80.74°F. Differences in salinity are also unlikely to be significant.
Continuous water changes: Continuous water changes, despite their name, are not necessarily performed every minute of every day. The distinguishing feature of these changes is that water is added at the same time that it is removed. The actual rate of addition can be high or low. Reef aquarists (myself included) most often perform these types of water changes with two matched pumps, one that removes the old water and one that adds the new water. Often these pumps are part of the same mechanism (such as two sets of tubing on a peristaltic pump or two heads on a diaphragm pump), but that is not a requirement. I use a dual head diaphragm pump capable of a maximum of 30 gallons per day for each head (a Reef Filler pump from Champion Lighting). In my setup, once I have a 44-gallon trash can full of new salt water, all I do to perform a 44 gallon or smaller water change is plug in the pump. The wastewater is sent down the drain. Sometimes I change 44 gallons in one shot, taking about a day and a half. Sometimes I pump for a few hours at a time, and then wait for a few days.
These changes are slightly less efficient than single batch water changes of the same total volume. A continuous water change of 30% exactly matches one batch 26% water change. As with very small batch water changes, these have the advantage of neither stressing the organisms (assuming the change is done reasonably slowly), nor altering the water level in the aquarium. The ease of doing such changes automatically also makes it far more likely that busy or lazy aquarists will actually do them.
