Aquarium Mixing Guide: Fish That Should Never Be Kept Together
Core Principles Before Mixing: Density and Space
The first step in mixing fish is not about which fish you want, but rather how large your aquarium is and how many fish it can accommodate. Overcrowding is the root cause of many mix failures.
Every fish needs a certain amount of swimming space, which is crucial for survival and water quality. Generally, small fish (such as tetras) need at least 1 liter of water per fish, medium-sized fish (like angelfish) require over 20 liters, and large fish (such as arowanas) need even larger water volumes. But this is not an absolute standard because fish body size, oxygen consumption, and waste production vary. A simple rule of thumb: each centimeter of fish body length needs at least 1 liter of water, but this only applies to small fish.
Besides water volume, consider the water layer each fish inhabits. The aquarium has upper, middle, and lower layers; different species occupy different levels. If all fish crowd the middle layer while the top and bottom remain empty, the mix will be unbalanced. For example, keep guppies in the upper layer, neon tetras in the middle, and corydoras at the bottom, which utilizes space effectively and reduces competition among fish.
High density also leads to rapid ammonia accumulation. Even with strong filtration, fish can become stressed and their immunity may drop. Therefore, always estimate the total fish count before mixing, and err on the side of fewer fish, especially since fish grow and need more space over time.
Matching Temperament and Size: Who Can Mix with Whom
The success of mixing largely depends on the temperament and size of the fish. Peaceful fish cannot be housed with aggressive ones, and fish with significant size differences should not be mixed because larger fish will eat smaller ones, or smaller fish may nip at larger ones.
Recommendations for Peaceful Fish Combinations
Peaceful small fish include tetras (neon tetras, cardinal tetras), platies, guppies, corydoras, and otocinclus. They usually thrive in planted tanks, but note that guppies are fast swimmers, and tetras prefer slightly acidic water. When mixing, ensure the pH is balanced; aim for neutral to slightly acidic conditions.
Medium-sized peaceful fish like angelfish (scalare), pearl gouramis, and dwarf gouramis can mix with other peaceful species, but angelfish become territorial during breeding, so provide ample hiding spots.
Aggressive Fish Species
Some fish are naturally pugnacious and should not be mixed with small fish. For instance, many cichlid species (like the African prince and snow cichlid) have strong territorial instincts and will chase and nip at others. Male bettas (Siamese fighting fish) attack each other and cannot be kept with other males, but they can be kept alone or with certain peaceful fish (like corydoras) if the tank is large enough.
Large fish such as arowanas, oscars, and blood parrots (referring to the hybrid cichlid) are robust and voracious; they will swallow any fish that fits in their mouths. If you mix them, they must be with fish of similar or larger size; otherwise, smaller fish will inevitably suffer.
Typical Forbidden Combinations
Below are some classic combinations that absolutely cannot be kept together.
Size Mismatch: Small Fish and Large Predators
For example, keeping neon tetras with arowanas, or guppies with oscars, where the latter will easily devour the former. Similarly, do not mix small fish with large catfish (like the redtail catfish), as they have huge mouths and will eat small fish without hesitation.
Same-Spirit Aggressive Fighters
Examples include two male bettas or two male dwarf cichlids. They will fight over territory until one is severely injured or dead. Many cichlids have the same issue; for instance, most African cichlids (Mbuna) cannot be kept with small fish because they not only chase but also dig sand and uproot plants.
Conflicting Environmental Requirements
Different fish require different temperatures, pH levels, and hardness. For instance, discus prefer high temperatures (above 28°C) and soft acidic water, while goldfish like cooler temperatures (18-24°C) and neutral to slightly alkaline water. Keeping goldfish and discus together will lead to one species failing to adapt. Moreover, saltwater and freshwater fish cannot be mixed; they must be kept separately.
Dietary Conflicts: Herbivorous Fish and Small Fish
Some fish are algae eaters or herbivores, such as goldfish and plecos (armored catfish). They may not actively attack small fish, but they might suck on the slime coat of weak or slow-moving fish, especially when they are stressed. In particular, when water quality is poor, plecos may attach to the body surface of large fish, causing infections. Therefore, avoid mixing plecos with weak fish.
Additional Points to Consider When Mixing
Besides the above principles, pay attention to the following.
Water Quality and Filtration
The more fish you have, the faster water quality deteriorates. Use a sufficiently powerful filter and perform regular water changes (one-third to one-quarter weekly). Avoid drastic temperature swings during water changes to prevent stress.
Feeding and Hiding Spots
When mixing, ensure every fish gets enough food, preventing aggressive fish from hogging. Use slow-sinking pellets or spread feeding points. Also, provide hiding spots like driftwood, plants, or rocks to give timid fish a refuge. But avoid overdecorating, as it may hinder swimming and cleaning.
Introduction Order
It is best to introduce peaceful or timid fish first, then more aggressive ones, allowing them to establish a new social structure. If you add aggressive fish first and then newcomers, the latter will often be attacked. Quarantine new fish before adding them to the main tank to prevent disease.
Density Revisited
Even with an ideal mix, density must be controlled. A common misconception is that small tanks can hold many small fish. In reality, the smaller the tank, the more unstable the water parameters, so the number of fish per liter should be reduced. A good guideline is no more than 3-4 small fish (under 3 cm in body length) per 10 liters of water.
Common Mixing Schemes for Reference
Here are a few beginner-friendly combinations.
Planted Tank Combo (Suitable for Planted Aquariums)
- Neon tetras + cardinal tetras + corydoras + otocinclus
- Features: All peaceful fish, adaptable to moderately hard, slightly acidic water; low mixing difficulty. Otocinclus and corydoras clean up leftover food and algae.
Medium-Sized Tank Combo
- Angelfish + pearl gouramis + clown loaches
- Angelfish and pearl gouramis are peaceful but have some territoriality; the tank should be large (at least 1 meter) with plants or driftwood to create zones.
Large Tank Combo (Over 1 Meter)
- Oscars + silver arowanas (note density and similar size)
- Be careful: these fish are voracious and produce a lot of waste; filtration must be strong. Not recommended for beginners.
Conclusion: The Key to Mixing Is Avoiding Conflict and Creating a Harmonious Ecosystem
In summary, mixing fish is not simply putting fish together; it is an art of balance. Always learn about the temperament, size, and water requirements of your target species, then plan density wisely, provide ample space and hiding spots, and consider introduction order and routine maintenance.
If you are unsure whether a particular fish can be mixed with your existing fish, consult professional resources or ask aquarium store experts to prevent fish casualties. Remember: it is better to keep fewer fish than to force an incompatible mix. May your aquarium be full of life and harmony.