Coral Reef, Torres Strait Islands, Australia

How to Keep a Coral Reef Aquarium: Physical Tank Parameters-Light, Heat, Salinity

If you say the word coral reef to someone, the picture that typically fills the bubble above their heads is one of beautiful azure water, dark shadows of coral just below the surface, and white sandy beaches that go on for miles. Its the image we see in print, photos, and as frequently in your favorite Mexican beer commercials depicting paradise. I am quite sure that those images, as well as the mystery of what is really going on below the surface, has lured more than one person to start a coral reef aquarium.  The reality though, is that the simple and beautiful view we see from above gives way to one of the planets most diverse and complex ecosystems below the surface.  Whether you keep a reef tank at home or manage a public facility, there are elements of nature which have to be recreated in a physical, chemical, and mechanical sense.  The goal being to try and simulate in captivity,  what our planet and the sun provide the worlds reefs with every day. While there are extreme examples of places corals can survive in the oceans, the following are some guidelines of what it typically takes to keep corals in captivity.  Specifically, we will discuss the physical parameters of light, heat, and salinity.

How Does Light Play an Important Role For Corals?

Most of the corals that are kept in reef aquariums, reside in clear shallow water less than 100ft deep in their ocean habitat.  The sunlight that penetrates to these depths is a critical factor in a corals survival as this is what provides them with the majority of their energy.  This energy is created in their tissues by symbiotic algae called zooxanthellae that utilize sunlight to produce sugars just like any other plant would do.  What is unique though is that they share some of this energy with their coral hosts.  In addition to this, most corals will also use their polyps to capture prey that supplements their diet in a sort of “surf and turf” balance.

In a reef tank, corals need for light is satisfied by positioning some type of lighting over top of the tank.  Typically these are controlled with some sort of timer that can simulate day, night, and sometimes even seasonal day length.  Lights come in many shapes and sizes and have evolved over the last 15 years from large tube fluorescent lights to metal halide bulbs and most recently LED light arrays.

Acropora Coral Bleaching in AustraliaTemperature, A Balancing Act In Nature As Well As In The Home Reef Aquarium

If you take a look at where coral reefs occur around the planet, you will notice a band that straddles the warmer waters from about 23 degrees north to 23 degrees south latitude.   Within this band occurs a range of temperatures that corals can tolerate.  It is tough to paint with a broad brush the temperature requirements of corals because there are some extreme examples of where they have been found.  For example, there are corals that live in cooler temperate latitudes outside the 23 degre band, as well as some that can exist in temperatures which would cause tissue bleaching in most other corals. In general though we see corals living within a temperature range from 70-82 degrees.

In reef aquariums, this same range will work to keep corals healthy.  That being said,  a range of 70-76 will result in slower calcification and metabolism while, the more common 76-82  will generally optimize most corals growth and health.  Above 84 or so and corals start to be stressed by temperature and at some range between there and around 90, most corals will hit a threshold where they expel their zooxanthellae in a last ditch effort for survival.  This gives way to the well known term known as coral bleaching.

An interesting aspect of the varied shapes of coral is that some are more temperature hardy compared to others. For instance it has been noted that in minor bleaching events, branching acropora colonies are more susceptible than mounding corals. One thought on this seems to be that the branching nature makes the acropora corals more influenced by surrounding water temperature. On the other hand, the mounding ball like shape of a Porites or Brain coral may afford them some ability to insulate themselves from warmer or colder temperatures for the duration event like natures igloo cooler.

Pass The Salt

Salinity is a measurement of the amount of salt in a given volume of water and is usually calculated as parts per thousand or ppt measurement at a given temperature.  In the ocean, salinity varies depending on whether you are near a coastal area where fresh water rivers lower salinity or in a places like the Mediterranean or Red Sea where evaporation makes the salinity higher.  In the reef aquarium , the measure of salinity is sometimes used, but more commonly reef keepers monitor specific gravity which is a measure of the density of a given sample of saltwater compared to a known substance.  All this sounds, and is very technical, but the most important takeaway from this is to make sure the device you are using to measure the salinity or specific gravity is accurate one that can also be calibrated.  All too often people have trouble keeping reef tanks for the simple reason that their salinity meter, gauge, or probe is not giving an accurate reading.   Typically in nature on a coral reef as well as in a reef aquarium, a salinity at or around 35 or a specific gravity of around 1.024-1.026 is what we try to achieve and keep constant.

We have covered Light, Heat, and Salinity which are 3 of the important physical parameters to pay attention to when keeping a reef aquarium, but there are certainly more.  In the subsequent articles in this series, we will dive into the mechanical and chemical makeup of a coral reef environment and how those parameters are important to the successfull keeping a reef aquarium.