
Maintaining and observing a peaceful aquarium can be a joy, but not every fish is suitable for a harmonious environment. Certain species can upset the balance of a tank due to their aggressive behavior, territorial conflicts, or unique care requirements that may not be ideal for community setups. Knowing which fish to steer clear of can help you save time, and effort, and avoid disappointment. Let’s discover 15 fish that may catch your eye but are best avoided in your tranquil aquarium, whether you’re just starting or have been in the hobby for a while.
1. Oscar Fish

Oscar fish are stunning and interactive, but they can be a handful in peaceful tanks. These fish are naturally aggressive and may bully or eat smaller companions. Moreover, they grow quickly, reaching up to 12 inches, requiring large tanks. Their messy eating habits often lead to poor water quality, demanding advanced filtration and regular maintenance. Additionally, their territorial nature makes them unsuitable for community setups. If their charm is irresistible, it’s best to house them in a species-only tank where their needs can be met without compromising the well-being of other fish.
2. Red-Tailed Shark

Red-tailed sharks are admired for their striking black body and red tail, but their aggressive behavior makes them a challenge. These territorial fish often harass or attack tank mates, especially those of similar size. They need plenty of room and cozy hiding spots to help them stay calm and avoid stress or aggression. Without these, their behavior can disrupt tank harmony. Additionally, they thrive in larger tanks with carefully selected, resilient companions. While experienced aquarists may manage their care effectively, for most hobbyists aiming for peace, it’s better to avoid this energetic swimmer and choose less territorial species.
3. Common Pleco

Common Plecos are often promoted as effective algae eaters, but their drawbacks outweigh their benefits. They can grow up to 24 inches, quickly outgrowing most home aquariums. Furthermore, their large size leads to competition with other bottom-dwelling species for food. They also tend to eat live plants, leaving tanks bare. Additionally, they produce significant waste, requiring robust filtration systems. While their appearance is appealing, their care needs make them challenging for peaceful setups. Smaller species like Bristlenose Plecos are a better choice for aquarists looking to maintain harmony and manage algae effectively.
4. Tiger Barbs

Tiger Barbs are lively with vibrant black-and-orange stripes, but their playful nature can cause problems. They’re notorious fin nippers, making them unsuitable for tanks with slow-moving or long-finned fish. Additionally, they thrive in groups of six or more, which helps reduce their aggression but requires a larger tank. Their active behavior can stress more peaceful species, disrupting tank harmony. While they are entertaining to watch, they aren’t ideal for beginners or aquarists seeking a calm community tank. For a peaceful aquarium, consider schooling fish like neon tetras or harlequin rasboras as alternatives.
5. Arowana

Arowanas are magnificent fish, but their size and care needs make them impractical for most home aquariums. These apex predators can grow over three feet long, requiring massive tanks to accommodate their size. Furthermore, they pose a risk to smaller fish, which may quickly become prey. Additionally, they require pristine water conditions and a specialized diet, making them demanding even for experienced aquarists. While their beauty is captivating, they are best suited for dedicated, species-specific tanks. If you’re not prepared for such responsibility, it’s better to admire them from afar.
6. Betta Fish in Community Tanks

Betta fish are breathtakingly beautiful but notoriously territorial, especially males. They often attack other bettas or nip at fish with long fins. Additionally, their preference for calm water makes them incompatible with active tank mates. While they can coexist with specific peaceful species, this requires careful planning and monitoring. Moreover, their solitary nature makes them thrive best in species-only tanks. For aquarists aiming for a tranquil community tank, it’s advisable to avoid adding bettas and instead focus on more compatible and sociable species that align with the desired environment.
7. Pacu Fish

Pacus are large, plant-eating fish that can grow up to three feet, making them unsuitable for most home aquariums. Additionally, their powerful teeth and appetite can wreak havoc in planted tanks, leaving them bare. These fish also produce significant waste, requiring advanced filtration and frequent maintenance. While they are often marketed as “friendly piranhas,” their size and care needs make them challenging to keep. They’re better suited for public aquariums or experienced aquarists with spacious setups. If you’re seeking manageable species for a peaceful tank, Pacus is not a practical choice.
8. Clown Loaches in Small Tanks

Clown Loaches are colorful and social, but they grow much larger than expected—up to 12 inches. Additionally, they are schooling fish that thrive in groups of five or more, necessitating a spacious tank. In small tanks, their activity levels and social needs can’t be met, leading to stress and health issues. Moreover, they require hiding spots and excellent water quality to thrive. While they are engaging and entertaining, smaller loach species like Kuhli Loaches are a better fit for peaceful tanks. For beginners or small setups, Clown Loaches should be avoided.
9. Green Spotted Pufferfish

Green Spotted Puffers are adorable but demanding fish that don’t suit most aquariums. They require brackish water—a mix of freshwater and saltwater—making them challenging to care for. Furthermore, their sharp teeth and territorial nature can lead to aggression, particularly toward smaller or slower tank mates. Additionally, they need a specialized diet to prevent overgrown teeth, adding complexity to their care. While they are fascinating in species-only setups, they are not ideal for community tanks or beginners seeking a peaceful environment. For simplicity, avoid puffers in mixed-species aquariums.
10. Goldfish in Tropical Tanks

Goldfish are a classic favorite but are incompatible with tropical tanks. They thrive in cooler water temperatures, while tropical fish require warmer conditions. Furthermore, goldfish are messy eaters, producing waste that can compromise water quality. Their slow movements and flowing fins also make them targets for fin nippers, causing stress and injuries. Additionally, their size and active nature demand a spacious tank with efficient filtration. While goldfish can be delightful pets, they require a dedicated, well-maintained setup. For tropical aquariums, it’s best to avoid adding goldfish to maintain compatibility.
11. Flowerhorn Cichlid

Flowerhorn Cichlids are admired for their vivid colors and unique forehead bumps, but they are highly territorial. These aggressive fish often attack other tank mates, regardless of size. Furthermore, they require large tanks with minimal decorations to reduce their hostility. Additionally, their specific care needs, such as water conditions and diet, make them challenging for beginners. While they can thrive in species-only tanks, they are unsuitable for community setups. For aquarists seeking a peaceful tank, Flowerhorn Cichlids are best avoided in favor of less aggressive, more manageable species.
12. Parrotfish

Parrotfish, known for their vibrant colors, often face challenges due to their hybrid nature. Their unique body shape can cause feeding and swimming difficulties. Additionally, they can become territorial, especially in smaller tanks, causing stress to other fish. Furthermore, their care needs, including dietary requirements and water conditions, demand attention and experience. While they bring color to an aquarium, they are better suited for experienced aquarists with dedicated setups. For beginners or those prioritizing harmony, choosing a more manageable, peaceful species is a better option than keeping Parrotfish.
13. Silver Dollars

Silver Dollars are peaceful schooling fish but can be problematic for planted tanks. These herbivores love munching on live plants, often stripping tanks of greenery. Additionally, they grow up to six inches and need spacious tanks to support their schooling behavior. While they are compatible with other peaceful species, their dietary habits and size make them less suitable for smaller tanks. Furthermore, their active swimming requires open spaces, making densely planted aquariums unsuitable. If you want a lush, tranquil setup, it’s best to avoid Silver Dollars and opt for species that don’t disturb plant life.
14. Giant Gourami

Giant Gouramis are gentle when young but grow to massive sizes—up to 28 inches—exceeding what most home aquariums can handle. Furthermore, as they mature, they may develop territorial tendencies, especially toward smaller or timid fish. Additionally, their waste output demands powerful filtration and consistent maintenance. While they are fascinating pets for experienced aquarists with large tanks, they are unsuitable for beginners or peaceful community tanks. For a calm and manageable aquarium, smaller gourami species like Pearl Gouramis are better choices. Giant Gouramis are better left to those with ample resources and expertise.
15. Electric Blue Jack Dempsey

Electric Blue Jack Dempseys are visually stunning but have an aggressive and territorial nature. These fish are especially combative during breeding and require plenty of hiding spots to reduce stress for themselves and their tank mates. Furthermore, they demand pristine water conditions and spacious tanks to thrive, which can be challenging for many hobbyists. While their vibrant colors make them appealing, their behavior and specific care needs make them unsuitable for community tanks. For a harmonious environment, they are better suited to dedicated setups rather than peaceful, mixed-species aquariums.