How to Choose the Right Freshwater Fish for Your Aquarium Setup

Choosing the right freshwater fish for your aquarium is one of the most important decisions you’ll make as a fishkeeper. It can be the difference between a peaceful, thriving aquatic ecosystem and a chaotic tank filled with stress, aggression, and disease.

It’s not just about picking the most colourful species or the ones you see on YouTube. Successful fishkeeping starts with understanding your tank — its size, setup, water parameters, and intended purpose — and then selecting fish compatible with both the environment and each other.

This article will guide you through the most important considerations when choosing freshwater fish for your aquarium setup, from tank size and aquascaping to fish temperament, stocking strategies, and common mistakes to avoid. Whether you’re building a nano tank, a community setup, or a centrepiece show tank, you’ll find practical tips to help you stock it wisely.

Understand Your Aquarium Setup First

Before you can choose the right fish, you need to understand the kind of environment you’re offering them. An aquarium isn’t just a glass box — its size, filtration, layout, and water chemistry determine which fish will thrive and which will struggle.

Tank Size Matters

Tank size is the first and most limiting factor. It determines the number of fish you can keep, their adult size, and their swimming behaviour.

• Small tanks (20–40 litres/5–10 gallons) can house bettas, shrimp, or small schools of nano fish.
  
• Medium tanks (50–100 litres/13–26 gallons) are suitable for community fish like guppies, platies, and corydoras.
  
• Large tanks (100+ liters/26+ gallons) offer flexibility for bigger fish like angelfish or rainbowfish and larger schools.
  

A common beginner mistake is to overstock small tanks. Always consider the adult size of your fish and their activity level. Overcrowding increases stress and reduces water quality, even with a good filter.

Also consider the tank’s shape, not just its volume. Long tanks provide more horizontal swimming space, which benefits active species like danios or rainbowfish. Tall tanks may look dramatic, but offer less usable space for schooling fish and tend to limit oxygen exchange at the surface.

Filtration and Flow

Different fish have different preferences for water movement, and this directly affects their health and comfort. Species like bettas, gouramis, and apistogrammas prefer calm, still water, while danios, hillstream loaches, and some barbs thrive in moderate to strong flow.

If your tank has strong filtration, consider using a pre-filter sponge, spray bar, or flow adjuster to reduce current in areas where slower swimmers dwell. In nano tanks, a sponge filter is often ideal — it provides gentle flow and biological filtration without stressing the fish.

Matching your filter and flow rate to your fish species helps prevent fin damage, stress, and fatigue.

Aquascape and Decorations

Aquascaping isn’t just about aesthetics — it directly affects fish behaviour, stress levels, and territory formation. Fish rely on hiding spots, visual barriers, and natural layout to feel safe and establish space.

• Shy or territorial species (like gouramis, apistogrammas, or dwarf cichlids) thrive with plenty of plants, rocks, and caves to break the line of sight.
  
• Schooling fish (such as tetras or rasboras) need open, horizontal swimming space to move in groups.
  
• Bottom dwellers (like corydoras or kuhli loaches) do best with soft substrates like sand and minimal sharp decor.
  

When planning your layout, research the natural habitat of your fish and aim to mimic it — whether that’s a planted riverbed, leaf-littered stream, or rocky shore. If you’d like to learn more about some popular beginner plants, check out our Java Fern, Anubias and Vallisneria care guides — they’re perfect for new aquarists.

Water Parameters

Water parameters — including temperature, pH, and hardness (GH/KH) — directly affect fish health and behaviour. These values vary based on your local tap water, bottled sources, or reverse osmosis (RO) systems. It’s important to research the ideal water parameters for each species you plan to keep, including pH, temperature, and hardness.

While many beginner fish can adapt to a range, others need specific conditions:

• Guppies and mollies prefer hard, alkaline water.
  
• Neon tetras and angelfish thrive in soft, acidic water.
  
• Corydoras and rasboras do well in neutral to slightly soft setups.
  

Always test your water before buying fish using a liquid test kit. It’s easier (and more sustainable) to choose fish that match your water than to constantly adjust parameters with additives.

Don’t Buy Fish Before Setting Up the Tank

It’s tempting to buy fish while shopping for your tank, but resist the urge. A brand-new aquarium is not ready for livestock until it’s fully cycled, meaning beneficial bacteria are present to safely process toxic ammonia and nitrites into less harmful nitrates.

Cycling typically takes 3–6 weeks and should be complete before introducing any fish. During this time, you can set up your aquascape, test parameters, and finalise your stocking plan.

Buying fish before cycling often leads to stress, illness, and even death, especially in fragile or sensitive species.

Types of Freshwater Fish and Their Needs

Understanding the behaviour and requirements of different types of freshwater fish is essential for building a peaceful aquarium. Some fish are active swimmers, others are shy bottom-dwellers or territorial centrepiece species. Choosing a mix that suits your tank layout — and each other — will prevent stress, aggression, and compatibility problems in the long run.

Types of Aquarium Fish Based on Behaviour

Schooling Fish

Schooling fish are social species that naturally live in large groups in the wild. When kept alone or in pairs, they often become stressed, skittish, or even aggressive. Common schooling fish include neon tetras, harlequin rasboras, zebra danios, and white cloud mountain minnows.

These fish thrive in groups of at least six, but larger groups are even better. They require horizontal swimming space and feel most comfortable in tanks with open middle areas and some plant cover along the edges.

Always stock schooling species in proper group sizes — it’s essential to their health and behaviour.

Territorial Fish

Territorial fish establish and defend specific areas of the tank, especially when mating or when space is limited. Common territorial species include male bettas, dwarf gouramis, apistogrammas, and many types of dwarf cichlids.

These fish may become aggressive toward others that look similar or enter their territory. To reduce tension, provide visual barriers such as tall plants, rocks, or driftwood that break the line of sight. This helps prevent chasing and stress.

Territorial fish thrive best in calm waters and environments with clearly defined zones, especially in smaller or shared tanks.

Bottom Dwellers

Bottom dwellers play a vital role in the aquarium ecosystem. These include Corydoras catfish, kuhli loaches, and bristlenose plecos. They spend most of their time near the substrate, scavenging for leftover food and algae. While they help reduce waste, they’re not a substitute for tank maintenance.

These fish need a soft, smooth substrate like sand or rounded gravel to prevent injury to their barbels or underbellies. Avoid sharp or jagged materials. Also, ensure they aren’t crowded out by messy decor on the tank floor — they need space to roam and forage peacefully.

Active Swimmers

Some freshwater fish are naturally fast, energetic swimmers that require plenty of open space. Common active species include zebra danios, rainbowfish, and many types of barbs. These fish are constantly on the move and may become frustrated or aggressive if kept in tanks that are too small or overly crowded.

Prioritise tank length over height — a long, horizontal tank allows them to dart and school comfortably. Avoid housing them with slow or delicate fish, as their energy can cause stress to more passive species.

An open midwater zone with limited obstacles is ideal for these swimmers.

Centerpiece Fish

Centrepiece fish are visually striking species that often become the focal point of a community tank. Popular examples include angelfish, dwarf gouramis, honey gouramis, and pearl gouramis. These fish usually swim slowly and gracefully in the middle to upper levels of the tank, making them ideal visual highlights.

While they are generally peaceful, some may become territorial, especially during breeding or if cramped. It’s best to pair them with peaceful schooling fish (like rasboras or tetras) and non-intrusive bottom dwellers. Avoid aggressive tank mates or fin-nippers that might harass your showpiece fish.

Community vs. Species-Only Tanks

When planning your aquarium, you’ll need to decide between a community tank, which includes multiple compatible species, or a species-only tank, which focuses on just one. Community tanks are great for variety and social dynamics, while species-only setups are ideal for sensitive, territorial, or breeding fish. Choosing the right approach depends on your fish preferences, tank size, and long-term goals for the setup.

What is a Community Tank?

A community tank is an aquarium designed to house multiple species of freshwater fish that can coexist peacefully. These tanks offer visual variety and interesting social dynamics, making them popular with beginners and experienced hobbyists alike.

For a successful community setup, choose species with similar temperaments, sizes, and water parameter requirements. Avoid aggressive or territorial species unless the tank is large and carefully partitioned.

Classic community fish include guppies, platies, tetras, corydoras, and dwarf gouramis. With thoughtful planning, a community tank can be peaceful, balanced, and full of life at every level of the aquarium.

When to Choose a Species-Only Tank

A species-only tank is designed to house a single species, either alone or in a group. This type of setup is ideal for fish that are territorial, sensitive, or have specialised care needs. It also allows aquarists to observe natural behaviours like breeding, nesting, or social hierarchies without interference.

Common examples include:

• Male bettas, which may fight with similar-looking tank mates
  
• Shell dwellers (e.g., Neolamprologus multifasciatus) that defend small territories
  
• Breeding pairs of cichlids, which can become aggressive during spawning
  

Species-only tanks are also great for aquarists who want to specialise, breed, or experiment with natural biotopes.

Compatibility and Temperament

Fish behaviour in aquariums is more complex than simple labels like peaceful or aggressive. Some fish are calm in one setup but hostile in another, depending on tank size, tank mates, or group dynamics. Understanding how to assess and manage compatibility is key to maintaining a peaceful, low-stress environment for all your fish. Avoiding common mistakes—like mixing incompatible fish—is just as important. This beginner’s guide covers what to watch out for.

Peaceful, Semi-Aggressive, and Aggressive

Fish are often categorised by temperament — peaceful, semi-aggressive, or aggressive — but these labels are best used as starting points, not strict rules.

• Peaceful fish like guppies, platies, and corydoras usually get along with a variety of species.
  
• Semi-aggressive fish, such as gouramis and some barbs, may nip fins or become territorial.
  
• Aggressive fish, including male bettas and many African cichlids, often require solitary or species-specific setups.
  

Behaviour is influenced by group size, layout, and tank mates. Always cross-reference temperament with tank requirements before choosing fish.

Avoiding Common Aggression Triggers

Even fish labelled as “peaceful” can become aggressive if their environment creates stress. Aggression is often a response to poor tank planning rather than a flaw in the fish’s temperament.

Common triggers include:

• Overcrowding – leads to stress and competition for territory
  
• Lack of hiding spots – increases tension and chasing
  
• Improper group sizes – e.g., schooling fish kept alone or in pairs
  
• Mixing incompatible species – like placing slow, long-finned fish with active fin-nippers
  

Prevent aggression by designing your tank layout carefully, stocking slowly, and always researching species’ social needs before adding them.

Fish That Should Be Kept Alone

Some freshwater fish are naturally aggressive, highly territorial, or simply stressed by the presence of other fish. These species are best kept alone or in highly controlled environments.

Common examples include:

• Male bettas – often attack fish with long fins or similar body shapes
  
• Pea puffers – small but notoriously aggressive, even toward snails
  
• Large predatory species – like some bichirs, oscars, or snakeheads
  

Keeping such fish solo isn’t a compromise — it allows them to display their full personality and natural behaviour without stress, injury, or competition.

Stocking Levels and the “1 Inch per Gallon” Rule

Why the Rule is Outdated

The traditional “1 inch of fish per gallon” rule is outdated and overly simplistic. It may work for small, similarly sized fish, but fails to account for:

• Bio-load (some fish produce much more waste)
  
• Swimming behaviour and activity level
  
• Body shape and bulk (a 2-inch pleco ≠ a 2-inch guppy)
  
• Tank shape and filtration power
  

Relying on this rule can lead to overstocking, stressed fish, and poor water quality. Modern fishkeeping requires a more thoughtful approach that considers compatibility, tank zones, and filtration.

Better Stocking Methods

Instead of relying on old stocking rules, use a layered approach that considers species needs, tank layout, and your maintenance capacity.

• Research the adult size and waste output of each fish
  
• Consider tank zones (top, mid, bottom) to prevent overcrowding in one area
  
• Ensure you have adequate filtration, and use live plants to help absorb excess nutrients
  
• Stock gradually, adding fish in phases to monitor water stability and compatibility
  

A well-planned stocking strategy creates a stable, low-stress environment and makes it easier to maintain water quality long-term.

Popular Fish Choices by Tank Size

Choosing fish that match your tank’s volume is essential for long-term health and harmony. Below are examples of freshwater species that thrive in small, medium, and large aquariums.

Small Tanks (20–40 Litres)

Ideal for nano setups or single-species tanks.

• Betta fish (solo) – vibrant and interactive
  
• Ember tetras – peaceful and colourful nano schoolers
  
• Chili rasboras – shy, tiny fish best in planted tanks
  
• Shrimp – cherry or Amano shrimp for cleanup and colour
  
• Sparkling gouramis – sound-producing and charming
  
• Pygmy corydoras – bottom-dwellers that prefer groups
  

Medium Tanks (50–100 Litres)

Good for a diverse, balanced community tanks.

• Guppies, platies, mollies – hardy livebearers with variety
  
• Dwarf gouramis – centrepiece species for midwater zones
  
• Harlequin rasboras – schooling fish with a calm temperament
  
• Kuhli loaches – shy, worm-like bottom dwellers
  
• Corydoras catfish – great in peaceful, planted tanks
  

Large Tanks (100+ Litres)

Allows for more complex communities or larger centrepiece fish.

• Angelfish – elegant and semi-aggressive
  
• Pearl gouramis – graceful, peaceful, and striking
  
• Rainbowfish – active, colourful swimmers
  
• Barbs – best kept in schools to reduce aggression
  
• Bristlenose plecos – manageable algae-eaters
  
• Tetras or danios – larger groups create impressive displays
  

Always ensure your tank conditions — including water quality, space, and decor — support the specific needs of your chosen species. It is important to mention as well that a well-maintained tank helps manage bio-load—learn how to clean your aquarium properly to keep your fish healthy.

👉 Want more ideas? Check out this list of beginner-friendly freshwater fish that are peaceful, hardy, and easy to care for.

Mistakes to Avoid When Choosing Fish

Choosing fish for your aquarium is exciting, but small mistakes during the planning phase can lead to stress, aggression, and even fish loss. Here are some of the most common errors new aquarists make — and how to avoid them.

Impulse Buying

Don’t let appearance or emotion guide your decisions at the store. Always research the adult size, behaviour, and tank needs of any species before purchasing. Many fish sold as “community safe” may grow too large, become territorial, or require different water conditions than your existing fish. And never buy fish before your tank is fully cycled. If you’re still figuring out how to begin, check out this step-by-step guide to setting up your first freshwater tank.

Overstocking

Beginners often try to fill the tank too quickly. Overstocking increases the bio-load and reduces swimming space, which can cause ammonia spikes, illness, or aggression. Stick to your stocking plan and allow your filter and bacteria colony to adjust gradually.

Mixing Incompatible Species

Placing peaceful fish with aggressive or overly active species rarely ends well. Always research temperament, size, and swimming style, and avoid combining fish with conflicting water parameter needs (e.g., hardwater guppies and softwater tetras).

Ignoring Water Parameters

Using tap water without testing is a recipe for problems. Test your source water’s pH, GH, and KH — then choose fish that thrive in those conditions. Adjusting water chemistry regularly is possible, but best left to advanced aquarists.

Summary: Plan First, Buy Later

Choosing the right freshwater fish for your aquarium setup isn’t about picking favourites at the store — it’s about building a peaceful, stable environment where your fish can thrive. The best choices are informed by tank size, water conditions, layout, and the social behaviour of the fish themselves.

Plan your stocking list carefully, cycle your tank, and build your aquascape around your fish’s needs, not the other way around. The reward? A harmonious, beautiful aquarium that runs smoothly and brings joy for years to come.