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 ceases. 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 the 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 rates).
A small amount of macro is unable to fully process a high concentration of dissolved nutrients regardless of the 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!
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