2nd Pillar of Reef Keeping – Chasing Coral Color & Growth

Ocean tides constantly flush the reef, washing away waste and bringing in nutrients, making water flow key in chasing coral growth and color to keep them in peak health.

So far, in this series we’ve covered the importance of lighting and providing the correct PAR level. In this 2nd Pillar of Reef Keeping article, we explore the importance of Water Flow.

Aqamai KPS Powerhead providing flow in a 20 gallon AIO aquarium

For so long water flow was thought of as simply making cool currents in the tank. But there’s some real science that backs up why flow is so important for corals, especially when they’re kept in an aquarium and is why it is our 2nd pillar in the foundation of reef keeping,. In their natural environment corals are exposed to many kinds of water movement. The flow patterns range from steady mild currents to cyclic wave action. One thing is for sure, the water movement never completely stops on the reef. Waves and tides constantly flush the reef, washing away waste products and bring in nutrients and suspended plankton. Since corals don’t crawl around and have limited tentacle range, they’re dependent on water currents to bring them food and nutrients while washing away gases and wastes.

The Boundary Layer

With target feeding and flow pumps, many reef aquarists think of water movement primarily as a way to keep their tanks free of detritus and sludge build-up behind live rock. But there’s so much more to water flow when it comes to keeping corals. All solid surfaces, including corals, are surrounded by a thin layer of “non-moving” water called the boundary layer which separates the corals from the surrounding water.

In nature, corals are surrounded by a 1-2 mm-thick boundary layer. Wave motion keeps the layer thin, allowing easier transfer of nutrients and wastes. Oxygen, CO2, wastes and nutrients have to pass through the boundary layer to reach the coral surface or be flushed away as waste products. The lower the flow, the thicker the boundary layer becomes, making it harder for the corals to carry on their natural respiration, nutrient absorption and waste processing.

Need for Water Flow

Corals rely on water currents to bring food and essential elements required for biologically critical processes. These processes include photosynthesis by zooxanthellae, carbonate skeleton building, reproduction, tissue repair and nutrient and dissolved gas exchange. There are a number of studies show zooxanthellae photosynthesis and skeletal growth are directly affected by water flow. Inadequate water flow in reef aquariums will reduce coral growth and inhibit their metabolism. But there’s more. Researchers discovered that some corals are covered with microscopic cilia that constantly beat, creating mini vortices that spin in the opposite direction of water flow. The miniature counter-current patterns to the boundary layer allowing better exchange between the coral surface and the water.

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Photo courtesy of MIT

An aquarium experiment with Acropora, Montipora, Seriatopora histrix, Protopalythoa, Palythoa and Discosoma was performed to measure the effect of water movement of coral health and growth. Vigorous water movement at the surface of the corals increased the transfer of nutrients and wastes by 400%!

Flow Rates within the Aquarium

Natural reefs are surrounded by turbulent water, which many powerhead manufacturers try to recreate with specialized programming. They offer customizable programming of different types of currents and various flow rates for throughout the day. The flow rate is typically determined by which type of coral you keep. For softies and LPS coral a 10-20 x tank volume per hour flow rate is recommended. For SPS coral a 20-40 x tank volume per hour flow rate is suggested.

Things to Keep in Mind

The 2nd pillar of reef keeping, water flow, is greatly affected by powerhead positioning, aquascape, pump cleanliness, and coral growth (as they mature). Coral growth can cause hobbyists to re-direct the tank’s water flow to accommodate these changes, or adding multiple powerheads will help get the flow into the areas lacking it.  The fact is corals must have good water movement to remain in peak health, when chasing color. The importance of water flow is a key factor when “Chasing Color” to keep coral in peak health for as long as you can keep them.

In Part 3 we’ll explore the third pillar of Chasing Color, Not Numbers: Nutrition.

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