Coral Bleaching in a Reef Tank: The Causes and What To Do

To save the corals in my aquarium from bleaching I spent a ton of time looking for answers. Now I’m ready to share the experience with you to keep your reef tank beautiful and colorful.

Corals bleach if they are stressed, which is mainly caused by excessive water temperature, lighting, overuse of fertilizers, and many other factors within the reef tank. The coral returns to a healthy state if you eliminate the cause of stress. For example, by lowering the elevated temperature.

If you’re interested in specific values and helpful tips from practice, read on. I will talk about the correct water composition, filtration, and more.

Why Do Corals Bleach?

Bleaching can be triggered by the following factors:

  • Insufficient or excessive lighting level, total darkness.
  • Too low or too high salinity. 
  • Stagnation of the water flow inside the tank, doldrums.
  • Physical injuries, traumas, or competitions among species.
  • Sustained abnormal temperatures or rapid changes. 1-2 degrees play a major role in the marine world.
  • Spread in values of alkalinity and pH.
  • The biological impact such as bacterias, algae, fungi.
  • Sun exposure, UV. A rapid increase or constantly affected.
  • Chemicals. Some substances do not completely decompose and affect the inhabitants of the tank. For example, low-quality water may contain heavy metals.

I took some information from this scientific article

A brief explanation of how corals discolor.

Almost all corals are transparent; they get the coloration from special microalgae that are incorporated into polyp tissue and supply the coral with up to 90% of all its energy. This is accomplished through photosynthesis, the process of obtaining energy from sunlight, which is why light is so important to corals. When an outside influence or change in habitat causes the coral to reject the algae, it loses the color. At this stage, the animal receives only 10% of the energy it needs. If conditions recover in the short term, the coral takes back the algae and becomes colored again. If the stress lasts too long, the coral starves until it dies.

The coral will gradually turn white

Here’s what you can do to protect the corals without doing any research beforehand:

  1. Biological filtration is the process of keeping the water free of harmful bacteria.  For example, fish waste such as ammonium and nitrites, poisoning the corals. Proper planning of the tank and filters can solve this problem. Make sure you have live rocks, sand, or a modern filter in the tank. I also suggest you create a schedule for partial water changes and stick to it without skips.
  2. pH between 8.1 to 8.3 is perfect, 8 to 8.4 is good.
  3. Alkalinity about 143 to 215 ppm. An absolute minimum is 125 ppm.
  4. Calcium normally ranges between 380 to 450 ppm (360 ppm is a critical threshold). 
  5. Ammonium is less than 0.1 ppm.
  6. Magnesium 1240 to 1340 ppm.
  7. Nitrate between 0 to 1 ppm is perfect, but 1 to 10 ppm also works.

If any parameter severely deviates, I suggest a partial water change. If this does not help, consider buying supplements. For example iodine solution or potassium powder.

Helpful info: PPM  — part per million, the same as mg/L.  10ppm = 10mg/L

I got some information from this scientific article

Reef Tank Water Quality & Coral Bleaching

Since you know the main factors, it’s time to learn how to deal with them. The thumb rule here is to keep an eye on crucial parameters and make sure they are stable. I check temperature and salinity once a day, for water flow, it’s twice a month. I also plan a deep check along with cleaning once a week. This is enough to keep corals healthy.

Now let’s go step by step.

  1. Go ahead, do a little research. Otherwise, you will spend time correcting mistakes. At this stage, refresh or get knowledge about your specific corals. Maybe some of them require a custom approach. Don’t forget to get info about equipment, reservoir, and chemicals you have. 
  1. Adjust crucial parameters. Set up the balance within the reservoir.
  • The perfect range for water temperature is 76° to 82°F. Rookies go over the acceptable limits all too regularly. This can happen due to high-intensity lighting or excessive heat in the room. If you notice an inaccuracy of a few degrees, don’t let it slide. Get back to the acceptable values right away. The temperature problem is the most common one. And the last thing you want is to underestimate it.
  • To cope with the salinity of water, it’ll be enough to buy a refractometer to easily measure it yourself. This is a special device to show how much salt is dissolved in the water. Normally you want a number around 1.024 – 1.025, but slight tolerances from 1.022 to 1.027 are not a big deal.
  • To measure water flow, connect the pump to a hose and insert it into a 1-gallon container. Time how long it takes to fill. Let’s say it took 10 seconds, so your pump may fill 6 gallons in a minute and 360 gallons in an hour. The normal flow for an aquarium is 10 times the volume per hour. For a 50-gallon tank, you need a pump that delivers 500 GPH. I suggest a closed-loop system for volumes greater than 45 gallons, for other cases a pump will do just fine.

Helpful info: GPH  — gallons per hour.

I got some information from this scientific article.

Tips on Lightning and Filtration to Prevent Coral Bleaching

To make lightning simple for you I divided it into 3 aspects:

Intensity.  PAR  — Photosynthetic Available Radiation  — is a common term for reefkeepers to measure the intensity of T5, bulbs, or LEDs. The average value is 100 to 250 but it varies depending on species. Soft corals and LPS benefit in a range of 50 to 150 PAR; Hard corals (stony, SPS) prefer numbers about  200 to 450 PAR.

Spectrum is a collection of different waves of energy emitted by light sources(T5, LED, Sun). Wavelength is measured in nanometers(nm). Corals can perceive light in 400 to 550nm and 620 to 700nm ranges. If the source does not fall in this range, then you are wasting energy (algae only absorb the blue and red part of the spectrum, which are in mentioned range) This part of the spectrum has beneficial effects on coral growth, development, and health.

Hint: Although PAR is a simple and convenient way to regulate, experienced aquarists use the PUR indicator. It eliminates the unnecessary parts of the PAR spectrum and leaves only the useful segments for the corals. For example, many species tolerate but dislike the red spectrum which is included in PAR but excluded from PUR  — Photosynthetically Usable Radiation.

Periods. Surely you understand that a coral needs light to live a full life. But what if it gets too much? Studies show that if there is an oversupply of light, the algae can’t handle the load and starts releasing a toxin. This leads to discoloration.

For me, the 14/10 ratio works. I expose the aquarium to 150 PAR for 14 hours, combining with 10 hours of complete darkness. Moonlight also counts as illumination. 

If you notice that soft tissues or polyps are retracting inside the skeleton, then you need to lower the light level urgently. The defense mechanism can handle the stress in the short term, but that does not mean that the coral is safe.

Filters. Usually, a filter is a place where beneficial bacteria live. Bio-wheels, balls, cartridges  — just to begin. Just like other parts of your aquarium, they need a clean-up. Careful handling of the filtration system will not kill beneficial bacteria, while excessive cleaning will put them at risk. 

Chemical filtration’s work principle is built on a substance called activated carbon. It absorbs toxic compounds and organic build-ups. Over time, the absorbency of carbon decreases. For effective water purification, it is necessary to renew the carbon every 2-4 weeks. 

Experience tip: I advise you to pick a large system. A multilevel filtration system prevents discoloration longer and is less demanding to maintain.

Related Questions

  1. How Are Zooxanthellae Related To Coral Bleaching?

Zooxanthellae are microalgae that coexist with the coral in a mutually beneficial relationship. In exchange for shelter, they help the coral get nutrients from sunlight. At higher temperatures, the activity of the algae increases, and they produce a toxin. The coral eats and spits out the microorganisms to avoid getting hurt, resulting in color loss.

  1. Do All Corals Bleach The Same Way?

The resistance to bleaching in corals differs depending on size, tissue thickness, and growth rate. Brain, tooth, and honeycomb corals can be a great option for newbie hobbyists. These species are among the most resistant to temperature changes. Birdsnest, fire, and staghorn corals lose color less often due to solid structure. It’s the perfect pitch for owners of the big tanks.

  1. Does Bleaching Affect Other Marine Animals?

Since bleaching is related to algae, very few species can be affected. Rare sponges may lose their tissue and bleach, as well as anemones which shrink in size but recover once the temperature is right. No need to worry specifically about the rest of the tank.

Bart Sprenkels

I have been keeping multiple aquariums since I was 18 years old. Just like many of you, I started with two goldfish but quickly learned they were not suitable for aquariums. Later, I switched to a tropical community tank and I also have two pet musk-turtles in a bigger aquarium. You can read more about me here.

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