Manipulating Nutrient Levels to Control Algal Growth

Ahhhh… A freshly set up and (just now) fully cycled marine aquarium. No algae. No slime. Clean,
shiny, beautiful.

Two weeks later? Maybe three? Algae. Slime. All over. You want your “old” tank back.
Particularly in newer set ups, these issues are pretty typical. Some aquarists believe something
is wrong if they don’t encounter a diatom bloom somewhere in the early stages of the system’s
development/maturation. Fact is, dissolved nutrient levels can swing wildly in younger systems.
Often, provided that good equipment is employed and good husbandry is practiced, these
problems simply peter out and go away.

Then there are some other systems. These cannot seem to ever get their act straight, with
nitrate, phosphate and silicate levels all over the place (and generally very high). Under these
conditions, one can only expect very heavy nuisance algae growth. As the algae dies or is
eaten/excreted it contributes to the detritus and dissolved organics pools. And more coming?


Bad algae can proliferate quite rapidly, particularly in recirculating aquarium systems where
nutrients (primarily nitrate released from the biofilter) accumulate to reach not just high, but
unnaturally high, concentrations. This can produce some especially aggressive algal blooms.
But, we’re talking about eliminating bad algae here. So, rather than focusing on removing that
which makes them thrive, let’s go for the throat and consider ways we can deprive them of the
very things that allow them to survive. How about starving them? No loss to you, since you
don’t really want all that nitrate anyway!

OK, right, get rid of excess nutrients to get rid of unwanted algal growth. Heard this all before?
You surely heard this simple version of the story. Be aware that controlling dissolved nutrient
levels is not always so straightforward.

Macros for your macro

Macroalgae, like any primary producer, uses various inorganic substances (i.e. nutrients) for
growth and development. Some of these substances are essential whereas others are not. The
essential nutrients can be distinguished as either micronutrients or macronutrients.

Micronutrients are required only in minute quantities (regardless of their abundance in the
environment); macronutrients, on the other hand, are always in really big demand. As they are
essential for survival, micronutrients are just as important as macronutrients.

In nature, populations of organisms are limited in size. This is because (even in the absence of
predators, disease, etc.) there is a finite amount of available resources to support survival and
reproduction. The first of these critical resources to become depleted (thereby halting
population gain) is referred to as the limiting resource. Particularly in oligotrophic (infertile)

habitats, the limiting resource very often is a nutrient. Nitrate and phosphate are the most
common limiting nutrients, though others (iron, silicate, etc.) may earn this distinction in
certain ecosystems. Nitrate tends to be limiting in freshwater environments while phosphate
tends to be limiting in marine environments.

As evident from the crystal-clear waters and “clean” rocks, coral reefs are extremely
oligotrophic habitats. And we have seen the effects of the high influx of nutrients (e.g. from
sewage discharge) into a reef environment: lots and lots of algal turfs and films. The same can
occur in a reef aquarium, where there is plenty of light, stuff to grow on and (most importantly)
a healthy dose of fertilizer.

Fighting fire with fire

This is where planted refugia come in. As strong competitors for nutrients, a growing bed of
macroalgae can soak up enough nitrate, phosphate etc. to starve out the bad algae (if not
prevent it from blooming in the first place). This natural method of water treatment has
continuously gained popularity over the last three decades simply because it works without

That is not to say that you can’t get a little extra out of your fuge by tweaking nutrient levels a
bit. Imagine that your macroalgae have pulled your nitrate concentration to undetectable levels
but extremely high phosphates remain. Now that macroalgae growth is limited and has all but
ceased, how are you going to get of the extra phosphate? Expensive chemical media? Laborious
and messy water changes?

Not necessarily… By adding (yes, adding) small, controlled doses of nitrate, you can get
macroalgal growth going with just a little bit more nitrate (potassium nitrate) until the
phosphate is gone. When the phosphate is finally gone, you can cut the nitrate. Harvest around
half of the crop, discard the harvested material and you’re done!

Yes, a whole generation of aquarists have succeeded using refugia for dissolved nutrient
management. But so many developments have occurred in recent times! For sure, newer and
cooler seaweeds keep showing up in the trade. And that’s pretty neat. But it will be really
exciting to see many newer and cooler ways to use seaweeds for this purpose. Arguably, using
them as vehicles for natural nutrient management will be the most important of these

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