Large Fish are major Waste Creators

Manipulating Nutrient Levels to Control Algal Growth

Ahhhh… A freshly set up and (just now) fully cycled marine aquarium.  Nutrient Free, 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.

Macro Nutrients 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 complication.

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!

Only the battle against Nutrients…

Live photoheterotrophic microbes can be especially helpful for long-term management in that they assimilate additional nitrate while consuming dissolved organic wastes released by the macros.

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 application.

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