fish for a marine tank

How to choose fish for a marine aquarium

Setting up a marine aquarium is an exciting endeavor, but it’s essential to carefully plan the inhabitants of your underwater world. Deciding what kind of animals to keep is the first step, closely followed by learning how to choose the right fish for a marine aquarium.

The most common mistake made by beginner aquarists is purchasing fish spontaneously without understanding their needs or behavior. This often leads to issues such as aggression, damage to corals, or even introducing venomous species that can pose risks to the aquarist. Selecting fish based solely on their appearance is not only irresponsible but can result in losses – both in fish and in your wallet.

What to consider then before buying fish for your marine aquarium? Well, to ensure your fish live healthy, stress-free lives, you must carefully think about several factors. Here’s what you need to know:

Size of the fish

Understand how large a species grows in the wild. While fish in captivity often don’t reach their full natural size, buying a young 5 cm fish that could grow up to 50 cm (such as Naso lituratus) for a small tank is a poor choice. Such fish are prone to stress, illness, and even deformities in inadequate spaces.

Tangs are large fish that require plenty of swimming space.
Tangs are large fish that require plenty of swimming space.

Fish temperament

With fish, as with people, each has its own unique character. Some are calm and prefer to stay in one corner of the aquarium. Others are very dynamic and can swim long distances. This behavior is also somewhat related to the size of the fish, as smaller species tend to be generally calmer. Fish that grow to a considerable size in nature possess the strength and energy to contend with water currents. In a small aquarium, such fast and robust fish may become fatigued, leading to stress for themselves and their tank mates.

Furthermore, the shape of the aquarium plays an important role. A 1,000-liter tank can be a cube measuring 1 m x 1 m x 1 m or be 4 m long. An example of fish that grow to a sizable size and swim energetically around the aquarium is the genus Paracanthurus, such as the Pacific Blue Tang. This fish not only swims quickly but can also carry sizable pieces of rock or unattached coral cuttings.

Another important consideration is combining fish with varying temperaments. In smaller aquariums (200-300 liters), it is essential to select fish whose temperaments are comparable. Otherwise, calmer fish may be dominated by the more dynamic species. In extreme cases, this can lead to death from malnutrition. In larger tanks, where there are more hiding places and ample space, this issue is less pronounced. However, introducing an onlooker like Forcipiger flavissimus into an aquarium with a group of highly active fish, even in a larger setup, may not be advisable.

Fish aggression

Even small fish can be quite feisty and disrupt the harmony of the tank. As a rule, aggression is typically directed at members of the same species or individuals of the same sex. However, aggression can also arise from similarities in shape or coloration, as these traits may trigger competitive or territorial behaviors.

In some cases, aggression within an aquarium intensifies during specific events, such as spawning. It’s also important to note that aggression is not always immediate. A potentially aggressive fish may initially appear calm after being introduced to the tank. However, once it becomes familiar with its new environment and the other inhabitants, it may begin to assert dominance and show its temper.

Pseudocheilinus hexataenia, fish in a marine aquarium
Pseudocheilinus hexataenia

A prime example is the small but strikingly colored Pseudocheilinus hexataenia. This fish can become so fixated on its chosen target that its relentless attacks may even lead to the prey’s death.

Other fish, such as Zebrasoma flavescens, might display aggression toward a new tank mate initially, chasing or harassing it. However, after a few weeks of persistent but non-lethal hostility, they often cease such behavior and coexist peacefully.

Zebrasoma flavescens
Zebrasoma flavescens

If a new aquarium inhabitant can endure the first two to three weeks of stress and potential harassment, it is likely to adapt and live harmoniously with the other fish. It’s also worth noting that aggression isn’t related to size. Even small fish can display surprising levels of hostility, sometimes even biting larger tank mates or the aquarist’s hand during maintenance.

Territoriality

Territoriality is closely tied to aggression. Setting aside the reasons for territorial behavior such as securing access to a mate or food two primary mechanisms can be identified.

The first mechanism applies to fish that inhabit a specific location, such as an anemone, and involves defending their nest. These fish remain unbothered as long as a potential threat stays at a distance. However, if a competitor comes too close to the anemone, the defending fish will initiate an attack to protect its territory. This behavior is typical of clownfish. In a sufficiently large aquarium, two pairs of clownfish can coexist peacefully with two anemones. However, if one anemone is removed, the displaced clownfish may be attacked by those residing in the remaining anemone if they venture too close.

The second mechanism of territoriality is less about location and more about the mere presence of a competitor. In this case, an attack or at least an aggressive display, such as fin twitching occurs as soon as the rival is detected, regardless of proximity.

Amphiprion ocellaris
Amphiprion ocellaris

In the territoriality of fish, the sense of smell plays a significant role. While in open reef waters it alerts fish to an approaching opponent, in the confined space of an aquarium, it becomes a constant source of stress. Additionally, territoriality arises from occupying similar feeding grounds. An example is the aforementioned Pseudocheilinus hexataenia, which feeds among rocks, consuming small crustaceans. Mandarins, such as Synchiropus splendidus, have similar feeding habits. If these two fish encounter each other among the rocks, it can lead to a fight, which is often won by the hexatenia.

Habitat

In their natural environments, fish settle where conditions are suitable for their survival and feeding. Many fish protect themselves at night in the nooks and crannies of rocks (such as anthias), among corals (like chromis), or by burying themselves in the sand (some wrasse). Therefore, when deciding on a specific fish, it is essential to consider its natural habitat ia a marine aquarium. If you have a minimalist aquarium with very few rocks, the availability of natural food may be insufficient to support a pair of mandarins. Especially if the fish are not particularly interested in frozen food.

Fish diet

Access to high-quality food is crucial for fish well-being, and today it is generally not a problem. However, the natural feeding instincts of fish present more of a challenge. While most fish will eventually learn to eat dry or frozen food, their innate food instincts can be revived, often even after several years in an aquarium.

Nature does not prioritize the beauty and appearance of corals as much as aquarists do, who often evaluate them based on the money spent. Therefore, aquarists must be aware of the natural diets of fish before introducing them into the marine aquarium. Numerous examples illustrate this: fish from the genus Centropyge may nibble on SPS polyps, the popular Acreichthys tomentosus may consume tube worms, and the aiptasia-eating Chaetodon species are quite keen on LPS polyps. I also know of an aquarium where Chelmon rostratus consumed an entire Tridacna maxima giant clam in a matter of hours, although it had not been touched for three years before.

Predation should also be mentioned, as some fish, such as Pterois volitans, can simply eat smaller tank mates.

When considering the food requirements of fish, you must also think about access to specialized food (usually live) for those fish known to be reluctant to accept substitute or frozen food. Such fish include the previously mentioned mandarins and chelmons, as well as seahorses and deepnosed pipefish. Often, during their first weeks or even months in the marine aquarium, these fish will not accept frozen food. Therefore, it is necessary to provide them with live food during this period.

Safety

You should be aware that some fish, such as members of the Scorpaenidae family, can pose dangers to humans. While these fish are generally distinctive and easy to recognize, there are venomous or poisonous species on the market that appear quite attractive. The popular Siganus vulpinus, often bought as an algae remedy, possesses a venom gland at the base of its fin rays. Some pufferfish or blowfish contain tetrodotoxin in their organs. Some beloved cowfish, like Lactoria cornuta, can release a neurotoxin into the water when under severe stress, posing risks to both aquarium inhabitants and humans. Given this information, it is essential to research fish thoroughly before purchasing them.

Siganus vulpinus
Siganus vulpinus

Natural resources

Marine aquaristics is a beautiful hobby that undoubtedly contributes to the understanding of coral reef ecosystems. However, it also impacts natural resources, which is why it is crucial to approach the purchase of aquarium inhabitants with care. First and foremost, many fish species are easily bred in captivity. With a little effort, you can even obtain offspring in home conditions. Buying fish propagated in captivity helps protect natural resources and reduces their extinction.

Bartek Stańczyk

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