What are Corals?
Corals belong to the family Cnidaria (nigh-dare-ee-uh), and are classified as animals. They have photosynthetic zooxanthellae, or little symbiotic algae, that live in their polyps, but each of those polyps is its own individual animal. Each polyp has a mouth and a rudimentary digestive system. Corals get their nutrients both from feeding from the water column and from the photosynthetic symbiotic algae living in their tissue.
There are many types of corals capable of living in your aquarium. Large polyp and small polyp stony corals secrete calcium from their polyps and form skeletons on which they live and grow. Soft corals are bound together with ever growing fleshy tissue. Colony forming corals like zoanthids and mushrooms form colonies of polyps like stony corals, but do so on a foreign medium, like a rock; they do not form their own skeleton.
Being a coral is biologically expensive. It takes quite a sum of energy to reproduce, build skeletons, and form new connective tissue. Corals obtain their energy in two ways. The first is to trap and consume plankton, or freely floating food materials in the water column. Corals accomplish this using barbed stinging cells on their polyps called nematocysts. They also intake nutrients from the zooxanthellae photosynthesizing in their polyps. If you want your corals to grow in size, color, and beauty, it's crucial to both expose them to full spectrum light, and feed them.
How Corals Feed
When the chemosensory organs, or smell sensors, of a polyp detect the chemicals that prey release in the water nearby,( i.e., when they smell prey), the polyps will release their nematocysts to catch the food items. The process starts by a flap on the surface of the polyp called the operculum opening up. When this happens, the filaments attached to the barbed, toxin-filled end of the nematocyst uncoils, and the nematocyst springs out from the capsule and into the water. The barbed end then attaches to the prey and releases toxins. The polyp’s tentacles then begin to move the prey toward the mouth of the polyp and the nematocysts recoil back into the capsule within the polyp.
There are a variety of coral foods on the market today, ranging from dehydrated powders to frozen zooplankton (tiny animals floating in the water) and phytoplankton (tiny plant cells floating in the water), as well as live copepods and amphipods.
powder (blend of multiple food types dehydrated in powder form)
copepods (tigger pods, calanus, blend/mix)
Choose the type of food that most interests you in terms of how you want to feed your coral. Live foods are the most hassle, but can also be the most rewarding experience. Dry and frozen foods are easy to feed and are clean. Some corals, such as small polyp stony corals (SPS corals) have such tiny mouths, only dry coral foods are practical feeding options.
When to Feed your Corals
Corals evolved to be crepuscular. This means they are active and feeding at dawn and dusk. This happens because of the increased amount of food available at dawn and dusk when the temperatures change in the shallow waters where corals live naturally. The temperature fluctuations cause swells and the increased movement in the water causes food items to rise and fall from the surface and ocean floor. If you ramp your lights up and down during the day, your corals will naturally release their nematocysts at dawn and dusk, signaling they are expecting food and ready to feed.
If you can’t feed at dawn or dusk, don’t worry. Corals are heavily chemoreceptive and will respond to food in the water regardless of the time. Some corals however should only be fed during certain times of the day; despite their name, sun polyps, for example, like being fed at night.
Regardless of when you feed your corals, try to feed them consistently, at the same time every day. Corals are intuitive animals and if you are consistent with your feeding, they will release nematocysts in anticipation of food around the time you normally feed them.
How to Feed Your Corals
Be sure to follow the directions on your individual food type for recommendations on how much and how often to feed. For the most part though, large polyp, small polyp, soft, and colony corals can be fed two ways; tank feeding or target feeding.
Tank feeding refers to simply mixing your food with some tank water then pouring the mixture into the aquarium. This method will release food into all parts of the tank, and most if not all polyps will interact with some of the food items. This method does result in more food not being captured by corals and ending up in the filter.
It’s a good idea to shut off your wavemakers, powerheads, and the filter pump. This prevents the food from being blown around too quickly for the coral to capture. If the food is suspended in the water column longer, your corals will have more time to release their nematocysts, making feeding more efficient.
Target feeding refers to concocting the same mixture you would with tank feeding, but using a syringe or pipette to apply that mixture directly to the immediate area around the corals you want to feed. This method is crucial for finicky eaters or corals that need extra food to grow. You must turn off your powerheads, filter, and wavemakers or the current will blow your food from the target coral almost immediately, giving it no time to feed.
Applying the recommended amount of food on the label to each coral you want to grow faster, or which are a little more sensitive, will ensure they remain healthy and beautiful.
Combine Both Methods
The best method, in my opinion, is to apply both methods to your feeding schedule. Start by shutting off your filter, powerheads, and wavemakers. Plug them all into the same strip plug if they're not controllable through an app, to make this easy. Then mix a quarter or half dose of food and pour into the tank. Wait for 5-10 minutes while your corals open up in response to the added food. Then take the other three quarters or half dose of food and syringe or pipette it directly onto the nematocysts of the corals you want to target feed. When in doubt, target feed as many corals you can. At the very least, be sure to feed your more sensitive species to ensure they thrive. Then wait another 5 to 10 minutes. Only turn your equipment back on when you notice most of the food has passed through the mouths of the polyps, you don’t want to blow away food items that your corals are actively trying to consume.
Don't hesitate to try different foods for your corals. Do your research and see which foods your corals like the most. Most soft corals, mushrooms, zoanthids, and gorgonians, for example, rely on phytoplankton for their nutrients. They can eat zooplankton, but the chemical makeup of phytoplankton is highly beneficial in comparison.
Also experiment with when you feed, how often, and how much. Be sure to watch your nitrate and organic nutrient levels after feedings. You don’t want to cause an algae bloom from overfeeding.
Most importantly, write it down. Taking notes on your feeding will give you valuable insight on what is working and what is not working. It is impossible to see patterns from memory alone, taking notes will spell those patterns out plainly in front of you. This will save time, money, and the lives of your aquatic friends.
For more on corals, check out this article on coral fragging from the Fish Tank Biologist Blog.
https://oceanservice.noaa.gov/education/tutorial_corals/coral01_intro.html#1 (accessed 11/21/2021)