Why Is My Acropora Turning Brown?

Has your Acropora started turning brown, and you are worried about your coral?

Acropora (Acros) are fundamental building corals, responsible for a large portion of the calcium carbonate in coral reef ecosystems, making them the “crown jewels” in the aquarium hobbyist world. 

They are a challenging SPS (small polyp stony) coral due to their care requirements, but taking on the challenge is a good thing for aquarium hobbyists like yourself! But, often the road to success has some small speed bumps, and your Acropora turning brown may be one of them you will face.

Sometimes, corals turn brown, and while it should not cause too much concern, you should definitely get to the bottom of the issue.

Why Do Acropora Corals Turn Brown?

The transportation process can be very stressful for your Acropora, particularly if they have traveled far to reach you. So, it may be that they may need more time to acclimate to their new home. You should always add new corals lower down in your aquarium, and then slowly move them higher up, or to their recommended placement once they have settled in. 

Depending on where your Acropora was sourced from, can also determine if it will arrive slightly brown. Freshly caught Acro frags can take a long time to fully color up, which is why many hobbyists opt for true aquacultured corals that have been grown in a captive environment for a long time. 

Selecting Acroporas that have been bred in captivity gives them more time to acclimate to aquarium conditions, which is why we always recommend buying from local hobbyists/local fish stores (LFS) that have successfully aquacultured Acropora corals.

However, if your Acropora Coral arrived healthily, and only recently has it gradually turned brown, then it may be down to them being a bit unhappy, which includes the following. 

Too Much Light

Your Acropora requires good lighting for photosynthesis, which has a little helping hand from microscopic algae that live within your coral’s tissues, called zooxanthellae. 

Zooxanthellae are pretty special. They play a vital role in the growth and productivity of your Acropora, because of their symbiotic relationship. You are probably thinking, “sym-what?” Well, to put it simply, they need one another to function. The zooxanthellae provide your Acro with oxygen and remove ammonia, nitrate, and other waste products, and in return, your Acro provides the algae with a safe place to live, so it can carry out photosynthesis. 

When Acros turn brown because there is too much light, the color change is due to the change in the distribution of zooxanthellae, a process called photoinhibition. This process is what can also drive the browning process because the photopigments are rearranged to shield from excessive lighting. 

This doesn’t always mean your lighting is too strong, it may be that your particular Acro was collected from a deeper area in the wild, so their natural color was developed at lower light levels. If this is the issue, try moving your Acropora coral slightly further down in the aquarium, aim for a pH level of around 8.3, and feed it some phytoplankton-based coral food if your nitrates and phosphates are under control. 

Too Little Light

If your Acro is turning brown because there is not enough lighting, increase the PAR slightly and check the spectrum your lighting fixture is providing. For Acropora corals, a PAR level of 250-500 is recommended, however, some Acros can tolerate levels up to 750 PAR. Before turning up the light intensity, always check the specific lighting requirements for your particular Acro species, and be careful you do not end up with a bleached Acropora.

To brighten your Acro and maintain optimal coral growth, you should have a full spectrum of light that is heavier towards the blues but also includes some reds, oranges, and yellows. 

Lighting Fixtures

It may also be the lighting fixture you have in your aquarium. While LED lighting is becoming more popular in the aquarium-keeping world, Acropora Corals prefer to grow under metal halides, or LED and metal halide hybrid lighting fixtures.

Elevated Nutrient Levels

Elevated nutrients (phosphate, nitrate, and alkalinity) create the perfect environment for the overproduction of zooxanthellae. As zooxanthellae cells take over, the chlorophyll cells which are naturally green, decline, allowing the zooxanthellae to display their natural pigments (golden-yellow to brown) inside your Acro, turning your Acro brown – a similar process to too much lighting. 

Regularly testing your water parameters is therefore essential to keep a close eye on nutrient levels (and alkalinity). 

Recovering Acropora That Has Turned Brown

Recovery depends on how severe the browning was, the original color of your Acro, and the cause. Some hobbyists see improvements within weeks, while others mention it can take several months for their Acro to regain full coloration. 

If your Acro has started turning brown, be patient. Patience is key when keeping corals, as they take time to acclimate to changing conditions.

How To Prevent Acropora Turning Brown

The best way to prevent your Acro from turning brown is tank stability. This involves keeping on top of those water changes, controlling safe nutrient levels, checking water parameters with a reliable testing kit, and controlling the water flow and lighting requirements. 

Tank stability is key when keeping corals, particularly Acroporas, as they are less forgiving than their LPS (large polyp stony) and softie neighbors. 

Maintaining Nutrient Levels 

Acropora corals require nitrate levels below 5 ppm, and phosphate levels no greater than 0.10 ppm. If they start increasing, perform a 25% water change immediately, across a few days, and keep testing the water until the levels are back to normal. 

If this is a continuous issue, it may be worth investing in a dosing pump – trust me, they make your life so much easier, especially for those holiday vacations!

Calcium, Alkalinity & Magnesium

Calcium and alkalinity are vital for your Acropora to grow its skeleton, a process called calcification for any coral nerds (guilty!). Keeping calcium and alkalinity levels stable is also important to prevent your Acropora from turning brown. It is recommended to maintain calcium levels between 400 and 420 ppm, and the alkalinity in the range of 8-9.5 dKH, although some Acro species can tolerate alkalinity levels slightly out of that range – so, always check your species requirements.

So, where does magnesium fit in all of this? 

Magnesium helps maintain calcium and alkalinity in your aquarium. This should read between 1250 and 1350 ppm when you check your water parameters. 


If you do not eat for a while, I’m sure you feel weak and may even start to lose some coloration. Well, your Acropora Coral can feel the same if they are underfed. However, feeding only helps prevent your Acropora from turning brown if nitrates and phosphates are not the issues. Overfeeding can cause excessive waste to accumulate in your aquarium, causing spikes in nutrient levels. 


Many factors can cause your Acropora to start turning brown, whether it be from too much or too little lighting, excessive nutrients, or stress from transportation. Do not panic, as it is usually an easy fix. Also, be patient as Acropora corals take some time to recover from browning events. 

As long as you remember tank stability, and the specific requirements for your Acropora coral, soon enough, your Acro will be back to its brightly colored self, and ready to show off to your friends!

  • Roy Lee

    I have an unhealthy obsession with reef keeping and maintaining successful tanks. If you haven't noticed from the website, I love everything related to saltwater tanks like coral, fish, and everything else in between.

    http://reeftankadvisor.com reeftankroy@gmail.com Lee Roy

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