How Long Do You Leave Your Refugium Light On?

One area of ongoing experimentation by aquarists is refugium
photoperiod. To date, no one has proven that a particular refugium lighting
regimen is ideal, at least for all systems. Fortunately, it doesn’t seem like
any are harmful! Planted refugia seem to “work” just fine as long as they
receive at least ten or twelve hours of intense light per day. Still, some aquarists
run their lights 24 hours/day without any apparent ill effect.

One method that is well worth mention here, though, uses a
so-called reverse photoperiod. The “reverse” part comes from the fact that the
refugium lights are timed to operate “on” while the main tank’s lights are
“off,” and vice-versa. The reason for this is that it balances (at least in
theory) water chemistry deviation that occurs as a result of the photosynthetic
activity of the plant life, both in the main tank and in the fuge.

As you very well may know, plants (including macroalgae)
take up carbon dioxide from their environment in order to store energy (i.e. make
sugar) as well as to build biomass. Drawing carbon dioxide from water alters
the carbonic acid balance such that raises pH. Plants only do this while
carrying out photosynthesis. The result is that (especially in a small, closed
system like an aquarium) sharp pH swings can occur between day and night
cycles.

In a reef tank loaded with hermatypic corals and clams, you
might have a lot of photosynthetic activity happening during the “lights on”
cycle; activity in the refugium can add to this if its lights run “on” concurrently.
However, if its lights are on alternately, the refugium can perform a
beneficial balancing act by stabilizing diurnal (i.e. day/night) pH values!

We only strongly suggest one thing; no matter what
light cycle you choose, be sure to use timers, because with macros, consistency
is certainly key to their health!