Why won’t my saltwater corals open up?

If your saltwater corals won’t open up or extend, it is likely for one of these five reasons.

• Water parameters: Salinity, alkalinity, calcium, and magnesium levels in your tank may be out of the acceptable ranges.
• Lighting: Too much light (photosynthetic active radiation) can be toxic to the coral, while too little light can slowly deteriorate and kill coral at a slower pace. 
• Water quality: Excessive nitrate, phosphate, ammonia levels, or unintended contaminants/pollutants such as lotions, oil, or soaps from your hands.
• Pests, Fungus, or Infections: Corals are living organisms and, therefore, susceptible to attack from pests such as flatworms, nudibranchs, fungi, and infections.
• Flow: Powerheads and filtration pumps can create hot spots where the flow rate is too much for the coral tissue, so they retract to protect themselves from tissue damage.

Even experienced reefers will have periods where their corals are not doing well.  It may be a pale or dull coloration, corals that won’t open or extend, or even tissue loss.  It may be just one or two corals, a particular type of coral, or it might extend to the entire reef tank.

A period of coral problems happens to almost everyone; even having corals die isn’t as rare as we would like.

My corals won’t open up after a water change; why?

When your corals won’t open up, water quality is the number one cause of coral issues.  The good news, in most cases, is that you can test your water parameters to determine what is wrong.  It is almost always water quality if you simultaneously have issues with various corals.

Salinity is the first parameter to check if your corals aren’t opening fully.  Corals have no mechanisms of osmoregulation.  Changes in salinity directly affect their metabolism and survival capacities.  The negative changes you see are the corals fighting to acclimate to the stress on their systems.  If you want to understand some of the science, you can download a paper by Ferrier and Gattuso here.

The salinity target for a coral reef tank is 35 parts per thousand.  You can be off by a point or two occasionally without drastic coral changes, but the discrepancy needs to be corrected to avoid coral deterioration in the long term. A great low-cost method of accurately testing for salinity is to use a refractometer. You can get the model I use cheaply through Amazon.com for less than $25.

You can easily adjust salinity by removing a measured amount of saltwater from the tank and replacing it with fresh RO/DI water if it is too high or by adding RO/DI to replace evaporated water over several days.  However, if you need to make big corrections, a series of large water changes may be necessary to correct the issue.

Corals won’t open up or extend

Often, your corals won’t open up or extend due to poor water parameters.  Alkalinity, calcium, and magnesium are critical elements to coral’s proper function and growth. When any of these three elements are outside the proper range, your corals will show negative effects. 

Alkalinity is often called the most important element, serving two major roles. The first role is maintaining the tank’s pH. The second provides carbonate and bicarbonate, the basis for how corals stay healthy and grow.  Managing a stable alkalinity level is one of the keys to having a successful reef tank and happy corals.  

Calcium is the second most essential element of a healthy reef aquarium. Corals, coralline algae, and other marine organisms (like invertebrates) require calcium for calcification and growth.  

Magnesium is the third essential element for healthy corals. In addition to biological functions, it prevents the excessive precipitation of calcium carbonate from the water. As mentioned above, calcium and alkalinity are essential to reef corals and organisms, and ensuring they are not lost to excessive precipitation is essential.

If your reef aquarium is low on alkalinity, calcium, or magnesium, you will see stunted coral growth, a loss of color, and corals closed or retracted.  Too much alkalinity, calcium, or magnesium will also cause precipitation, coral damage, and even damage to your aquarium equipment.

I use Hanna Checkers to test my alkalinity, calcium, and phosphate levels for digital readout accuracy. You can check out the full line here.  

I have an article describing handling high alkalinity and imbalances between alkalinity, calcium, and magnesium.

Water parameters that can keep your corals from opening-up

It can be easy to fall into habits that allow the slow deterioration of water quality from poor feeding or maintenance practices. Unintended contaminants/pollutants such as lotions, oil, or soaps from your hands can also deteriorate your water quality.  Aerosols used near the tank can also add pollutants.

If you think your reef tank might have harmful contaminants or pollutants in the water, it is a great time to break out the carbon!  Change out your carbon or add carbon into your media reactor, throw a bag in the media tower, or whatever method you use carbon, be sure to get some fresh carbon into the filter system to pull those pollutants out of the water.

If your corals won’t open up after the carbon, it’s time to begin doing water changes.  Usually, three to four 30% water changes over a few days will do the trick. If you suspect a particularly harmful accidental contaminant and things are rapidly deteriorating, you may have to perform 50% or even 90% water changes.  It’s better to do huge water changes than to watch your tank wither and die.  Just make sure the water is properly mixed and heated to put as little stress on your livestock as possible.

I have an article here on how to set up an easy-to-manage reef tank maintenance schedule.

Nutrient levels can keep your corals from extending

If your nitrate or phosphate levels are out of range, it can affect your coral’s polyp extension and may be why your corals won’t open up.  Overfeeding and poor tank maintenance are sure ways to spike nitrate and phosphate levels.  Conversely, chemical filter media and other aggressive nutrient control methods can cause nitrate and phosphate levels to bottom out.

Knee-jerk reactions to high levels of nutrients do not take the place of proper reef aquarium management.  Feeding your livestock sensibly, limiting the use of organic-based supplements and additives, using a high-quality synthetic aquarium salt blend, performing regular water changes, and replacing evaporated water with reverse osmosis/deionized water are important aspects of nutrient control.

If you do not manage your reef tank nutrient parameters, your corals will not grow properly, they won’t extend polyps and tentacles, and they will not appear colorful and healthy.

Lighting can stop corals from opening-up

Too much light (photosynthetic active radiation) can be toxic to coral.  One sure method to quickly kill a new coral is to place it in a hot spot with too much light.  Too much lighting can cause a coral to shrink and retract polyps or bleach white. A coral can die in a matter of days with too much light.

Not having enough light may take weeks to show signs of coral distress and months to die.  Corals often show insufficient light by stretching out and reaching for the light or turning brown.

I like to start new corals very low in the tank to get them acclimated over time.  Understand what PAR the coral type typically thrives in before relocating it to its final spot.  Soft corals and LPS, like a lower PAR, are usually placed low to the middle in a reef tank.  SPS (Small Polyp Stony) coral typically enjoy higher PAR and are usually placed middle to high in the aquarium.  I have an article here on LED lighting and PAR recommendations.

If a coral struggles in its new location, move it to a lower-light tank area, like the sides or bottom.  It may take up to a couple of weeks to recover before you should move it again.  

Pests, Fungi, or Infections can keep your corals from opening-up 

Corals are living organisms susceptible to attack from pests such as flatworms, nudibranchs, fungi, and infections.

These kinds of issues usually only affect a single coral or type of coral.  If you suspect any of these culprits are damaging the coral in your reef tank, it is time to remove the coral and dip it carefully. 

Various dips kill pests, such as flatworms, nudibranchs, and zoanthid-eating spiders.  Dips will also help heal coral of rapid and slow tissue necrosis, bacterial infections, and more.  If you properly diagnose the problem, the dip will likely return your coral to its previously happy self.  The before and after polyp and tentacle extension is often dramatic when a pest or disease-ridden coral has been dipped and is returned to the tank.

Coral RX Coral Dip from Coral RX is one of my favorite dips that will kill pests and cure infections. You can get it here for a great price.

Why are my soft corals not opening up or extending?

Corals with soft tissue, such as large polyps or tentacles, are often more sensitive to flow from powerheads and filtration pumps than corals with smaller polyps.  Each coral type acclimates best in a particular type of flow.  Soft corals and LPS (Large Polyp Stony) corals typically prefer moderate to low flow areas of the tank. A rhythmic pulsating flow pattern typically works well with soft corals.

Water flow is normally pretty stable in a reef aquarium unless you are constantly fiddling with your powerheads’ placement or adjustments.  If you have a coral that has been doing fine where it’s at, and suddenly it’s not opening or extending, it is likely a parameter issue and not water flow.

If you have added a new coral and it’s not opening or extending after a couple of days, you should try it in an aquarium area with less flow to see if the flow rate or pattern is the problem.

Frequently Asked Coral Questions

1. Why aren’t my saltwater corals opening up?
   • There could be various reasons why your corals are not opening up. Some common reasons include poor water quality, inadequate lighting, lack of proper water flow, and improper feeding.
2. What could be causing my corals to remain closed?
   • Corals can remain closed due to stress caused by environmental changes, such as fluctuations in water temperature, pH levels, and salinity. Also, pests, disease, and poor lighting can cause coral stress.
3. Is there anything I can do to encourage my corals to open up?
   • You can try to improve the water quality, adjust the lighting or feeding schedule, and increase water flow around the corals. Additionally, check for any signs of pests or disease, and take corrective measures if necessary.
4. How do I know if my corals are healthy?
   • Healthy corals are characterized by bright colors, fully extended polyps, and a firm, plump appearance. If your corals look pale, shriveled, or are not fully extended, they may be unhealthy.
5. Can lighting affect coral behavior?
   • Yes, lighting is an essential factor in the growth and health of the coral. Different coral species have different lighting requirements, so matching the lighting to the specific needs of the corals in your tank is essential.
6. Could water quality be causing my corals to remain closed?
   • Poor water quality can significantly cause coral stress, resulting in closed or unhealthy corals. Ensure you maintain optimal pH, salinity, and temperature levels, and regularly test your water for pollutants.
7. What are some common reasons for coral stress?
   • Common causes of coral stress include changes in water chemistry, temperature, and lighting. Pests, disease, and overcrowding in the tank can also cause stress.
8. How long should I wait before determining that my corals are not going to open up?
   • It depends on the specific coral species and the conditions causing stress. However, if your corals have remained closed for over a few days, it’s time to investigate and take corrective measures.
9. Are there any special feeding requirements for corals?
   • Yes, corals require proper feeding to maintain their health and growth. Most corals are photosynthetic and rely on light energy to produce food. However, some corals require supplemental feeding with phytoplankton, zooplankton, or other specialized feeds.
10. Can overcrowding in my tank cause my corals to stay closed?
    • Yes, overcrowding can cause stress to corals, leading to closed or unhealthy corals. Ensure you have an adequate tank size for your coral species and maintain proper water flow and filtration.