A refugium must be large enough to function properly. That
is, not just keep a bed of macroalgae alive, but also provide enough space to
allow for continuous growth. Without continuous growth, nitrate and phosphate
removal cease. Worse, if an overgrown mass starts to die back, you could
actually experience a decline in water quality.
So, how large should a refugium be? As much as you might
love a simple answer to that question, there really isn’t one (not one that is
correct, anyway!). The answer to this depends upon many factors, and every
aquarium system is unique. How much nitrate/phosphate is produced per gallon
per day in this system? Is the refugium lighting supporting maximal photosynthetic
rates? What is the rate of water flow through the refugium? What type(s)
of macro are you using? How much/often do you harvest from the standing crop?
If you want to operate a refugium primarily for efficient nutrient
removal, you’ll need to use chlorophytes
(green algae), as they exhibit the fastest growth rates. Some aquarists might
opt for Ulva instead of Chaetomorpha, as the former is far more
useful as a live veggie food than the latter. But you’ll additionally have to
consider flow rates; to get maximum nutrient removal, you’ll need to turn over
the total refugium water volume per some amount of time, right? Chaetomorpha can physically withstand significantly higher flow rates than Ulva, for
example; this means that you’ll need a bigger grow space for Ulva (that
is, to get proper turnover rates through the refugium without super high flow
A small amount of macro is unable to fully process a high concentration
of dissolved nutrients regardless of turnover rate. This means the bed will
have to be of a minimal size--and actively growing. Thus, in order to provide
enough space for a bed and space to grow into, the fuge should be at
least (very roughly) a third of the size of the main tank, depending
much upon actual bioload of that unique system. Can you see why those dinky
hang-on-the-back refugia are as laughable as they are cute?
Honestly, there is no refugium that is too big. And
that’s the only thing we can say for certain. That’s why when we hear “how big”
for refugia, we say “way big.” As in, as large as possible!