Coral-Only Tank (Fishless Reef) Guide

Tappinen, CC BY-SA 3.0, via Wikimedia Commons
Tappinen, CC BY-SA 3.0, via Wikimedia Commons

Coral-Only Tank (Fishless Reef) Guide


If you are interested in building a coral-only reef tank, this article will provide the guidance you need. Either if you like the clean look of a tank filled with corals only or you simply don’t want to worry about having fish, this option is possible and not that complicated.

You’ll learn everything you need to know about what makes a coral-only (fishless) reef tank different from a regular reef tank that also has fish in it.

What is a coral-only reef tank?

A coral-only reef tank is a reef tank that includes only corals. This coral-only approach typically refers to the absence of fish, but some might prefer to not include anything in the tank besides corals, meaning no invertebrates either.

A coral-only reef tank requires almost the same maintenance as a regular reef tank, but there are some differences that you need to consider when making this choice.

The Main Differences Between Coral-Only Reef Tanks and Reef Tanks with Fish

In nature, fish and corals create an environmental harmony that you need to compensate for when you take the fish element out of the tank.

Fish provide a food source

Fish excrements are some of the biggest sources of nutrients for corals in reef tanks, and removing them from your tank means you’ll need to pay closer attention to coral nutrition.

The food generated through photosynthesis by the algae present in the coral’s tissue, the zooxanthellae, might be enough to keep the corals alive but not enough to keep them thriving. If photosynthesis would have been enough for corals, they wouldn’t also feed through polyps.

Fish waste is a big part of the organic matter that serves as food for filter feeders such as corals.

Coral-only reef tanks require feeding more often than coral tanks with fish in them. You need to feed your corals weekly if you want to keep them happy without fish.

But your saltwater tank will be cleaner. Not having an abundance of fish comes with its advantages. One of the reasons people want to have a coral-only reef tank is because they are tired of seeing fish waste in their tanks.

A coral-only reef tank will be much cleaner and provide better aesthetics.

Fish balance nutrients

Coral reefs occur in waters that are naturally low in nitrogen and phosphorus. A certain amount of these nutrients is essential for coral growth, while too much can increase the chance of coral disease and death.

Fish help supply the reef tank with nitrates and phosphates, which are essential for coral health and growth if they are in the right amount.

If you keep a coral-only reef tank, you’ll have to keep an eye out on nutrients, properly dosing and testing your water regularly.

Getting rid of pests and algae

Fish are more than a food source in a reef tank. They are also cleaners, eating the algae before it gets too big and getting rid of annoying pests before they affect your corals.

However, they are not the only ones capable of cleaning your reef tank.

Invertebrates such as snails, shrimps, crabs or starfish, are more than capable at keeping your reef tank clean. They munch on algae, detritus or cyanobacteria that will affect your reef tank if left unchecked.

Having a CUC (Clean-up crew) made up of invertebrates is essential for reef tanks, but even more so for coral-only reef tanks.

Make sure you include a healthy dose of invertebrates in your tank and you shouldn’t feel the absence of fish.

Frequently Asked Questions About Coral-Only Reef Tanks

Do you need to cycle a coral-only tank?

Cycling is a term referring to the Nitrogen cycle, a process where beneficial bacteria build up in your tank. You need this bacteria to break down harmful ammonia (NH3) into nitrite (NO2) and then the safer nitrate (NO3).

Yes, you need to cycle your reef tank even if you plan on having only corals. Ammonia will be harmful to your corals or any invertebrates you add to it if you don’t.

Do you need a protein skimmer in a coral-only tank?

Yes, you would need one. Since you’ll feed the corals to make up for the lack of nutrients in the water, uneaten food will rot if you don’t have a skimmer.

What do you feed coral-only tanks?

The food you can feed your corals depends on what types of corals you have. LPS corals can eat macroscopic or larger prey, while SPS corals can only feed on smaller prey.

It’s important to offer a variety of foods to provide the corals with all the nutrients they need. You can prepare food from a mix of krill, diced small fish, phytoplankton, thawed frozen plankton, and pieces of clam, shrimp, or squid.

Or you can use a food mix that you can purchase from reef tank shops.

Is filtration still needed for coral-only tanks?

Yes, chemical filtration with carbon media is still recommended, as corals release a substantial amount of organics such as proteins, hormones, organic acids, phenols, and carbohydrates that need to be cleaned from the water.

The metabolic waste or organics in the water can pose a threat to the corals and invertebrates living in it. While fish increase the amount of organic waste in the water, they are not the only producers of it.

What is the easiest coral to keep?

Soft corals are fantastic for beginners, as they are easy to keep and tolerant to water quality. These corals will add dazzling colors and amazing movement to your reef tank. They are also available aquacultured, which means they are hardier than wild-caught species.

Here are the top 5 easiest corals to keep:

    1. Xenia Polyps – One of the easiest corals to start with. Pulsing (Pom Pom) Xenia is one of the most favorite for its fast growth, minimum lighting requirement, and tolerance to dirty tanks.
    2. Zoanthid Polyps – Another easy to keep, hardy and colorful coral. You can find it in a multitude of colors, they easily reproduce in an aquarium and require minimum water flow and lighting.
    3. Green Star Polyps – An encrusting soft coral that will grow on your rocks, substrate, and sometimes even the wall of your aquarium. It grows fast, being able to overtake almost any type of hard surface, and it’s extremely hardy.
    4. Toadstool & Leather Corals – They grow in the shape of a mushroom. The Toadstool variety has a brown base with white polyps, while Leather Corals might feature green polyps. When these corals are happy, their long-thin polyps will extend from their surface, indicating that things are going well in your aquarium. They can grow very large in a short amount of time.
    5. Clove Polyps – They lack the colors of other corals mentioned, but they bring a lot of movement in the tank. Their long beautiful tentacles wave in the water, and they easily grow and encrust on other rocks. Because of the long polyps, they can get stung by other corals, therefore give them a bit of space when placing them. Clove Polyps grow fast and can endure in most aquariums.


Having a coral-only reef tank is possible if you make a couple of adjustments. Keeping a coral-only reef tank isn’t that different from a regular reef tank, however, you need to compensate for the lack of fish.

You can easily compensate for the lack of nutrients by feeding your corals more often and you can keep a closer eye on the nitrates and phosphates levels to not get out of hand.

You also need to find a way to keep the algae levels to a minimum since the coral food will also feed the algae inside your tank. Having a CUC is highly recommended.

If you love the cleaner look of coral-only reef tanks and you are willing to make the necessary adjustments, this is a great way to build a reef tank.

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