Acropora (Acro) corals are likely to originate from different conditions than those in your aquarium. Even if you have bought your Acro frag from a local hobbyist/local fish store (LFS) that has successfully aquacultured Acropora corals, it is very unlikely they have the exact water quality, lighting, and water flow conditions as you.
While some Acro corals can tolerate minor fluctuations in water conditions before turning white, others can quickly change color overnight, often leaving aquarium hobbyists like yourself very concerned for their health.
5 Reasons Acropora Corals Turn White
Transportation is a stressful time for your Acro, and occasionally, corals do not survive the journey. If you have removed your coral from its packaging and noticed that it is completely white, call up the store or person you bought it from – you may be able to get a refund or a new one sent to you!
Another stressor is acclimation. When your Acropora coral arrives, you should dip it in a coral-safe dip like CoralRX to remove any pests. Acropora corals are susceptible to Acropora Eating Flatworms (AEFW), which once infected, will strip their tissues as these worms munch their entire way around your coral, therefore, dipping is a must!
When a new coral arrives, it is a very exciting time. You probably have had the perfect space for a while and know exactly where your Acro is going to go. But wait a minute! When you add your Acropora to your aquarium, place it near the bottom and slowly move it up to its final destination. This gives your coral time to acclimate to its new home.
2. Growth Tips
Acroporas turning white is not always bad news. As Acros grow, their tips become white. If it is only the tips on your Acropora turning white and your water parameters are stable, then there should be nothing to worry about, in fact, you should be super excited that your Acro is growing healthily!
The reason behind your Acropora’s white growth tips is the absence of zooxanthellae in your coral’s new calcium carbonate skeleton. As zooxanthellae are responsible for photosynthesis inside your Acro’s polyp, zooxanthellae enhance calcification rates and provide pigmentation for SPS corals. So, if you ever experience your Acropora turning brown, then you know the zooxanthellae are being overproduced.
As already mentioned, white tips should be no cause for concern, however, if the white coloration starts spreading further down your Acropora’s branch without stripping the tissue, you could be bleaching it.
3. Coral Bleaching
Coral bleaching is nothing we haven’t heard of before. Because of global climate change, mass coral bleaching events have happened in Australia due to elevated sea temperatures and sunlight overexposure.
If your aquarium temperature is too high, your Acro will expel the zooxanthellae from its tissues, turning ghostly white. Do not fear yet, as bleached corals are not dead corals (yet). Most corals can survive bleaching events, and in aquariums like yours, you can change the environmental conditions to help recovery fairly easily.
If your Acropora is becoming bleached, turn down the lighting very slowly. Any changes must be done steadily to reduce stress levels for your coral. Acropora corals thrive under metal halide lighting with a full spectrum and a PAR range between 200 and 450. However, some Acros can tolerate PAR levels up to 750, so always check which Acro species you have, so you can hit its sweet spot without bleaching it.
4. Alkalinity Burn (Burnt Tips)
Alkalinity burn, often referred to as “burnt tips”, look similar to white tips, but your Acro’s flesh around its tip will have some tissue loss. This happens when your nutrient levels significantly drop and alkalinity skyrockets.
Alkalinity and calcium are both responsible for your Acro to build its calcium carbonate skeleton. When alkalinity levels are through the roof and nutrient levels are depleted, your coral will be unable to grow its tissues at the same rate as its skeleton.
As alkalinity swings can be fatal for your Acropora coral, it is important to tackle that issue before focusing on altering the light intensity.
Most of the time, if you spot your Acros burnt tips early enough, they will sort themselves out over time. First, check your water parameters, and adjust where needed. As with any changes inside your aquarium, remember they must be done gradually, without changing too many water parameters at the same time.
5. Slow/Rapid Tissue Necrosis
If your Acro’s tips are turning white and are covered in algae, then it is probably not growth tips, but the dreaded slow or rapid tissue necrosis (S/RTN). Algae don’t grow on healthy, living tissue, therefore, algae growth on white tips that have an uneven surface, is a good indication your Acro is suffering from S/RTN.
If you suspect STN or RTN, then you must test the water immediately and check the parameters are within range. As it can be spread between corals in water, quarantining your Acro before placing it in your main aquarium when it arrives reduces the chances of neighboring corals also becoming affected.
As the name suggests, STN takes some time, usually days, but sometimes months. When you notice STN you must take it seriously, as once infected, the battle begins to save your beloved Acro. With STN, you will notice other white areas on your Acropora, typically at the base, as the necrosis likes to start there and work its way up. As the necrosis worsens, you will start to see your Acros skeleton exposed.
Tissue necrosis comes from microscopic protozoans that live in unbalanced conditions, so any spikes in your water conditions, particularly temperature, create the perfect environment for them to infect your Acropora.
If your Acro is suffering from S/RTN, frag off any infected or dead tissue. If it has spread over your entire coral, you may need to think about throwing it out and buying a new one.
To prevent it from happening, frequently test your aquarium water and carry out the pre-treatment advice for new corals (acclimation and quarantine).
When Acropora corals turn white, you may just be noticing their growth tips, which is a good thing! However, there can be several issues that cause concern such as stress, alkalinity burns, coral bleaching, and slow/rapid tissue necrosis.
Hopefully, white tips are the only whitening you will see on your Acro, which is why acclimating and maintaining a stable environment is key to successfully keeping your Acropora coral happy and healthy.