How to Choose the Right Fish Tank Size for Your Home

Setting up a home aquarium is an exciting and rewarding project — but before you start choosing colourful fish or decorative plants, the first (and most important) decision you’ll make is what size fish tank to buy. The size of your aquarium will influence everything else: the type of fish you can keep, how healthy your ecosystem will be, and even how easy it is to maintain.

In this guide, with expert advice from the friendly folk at Complete Aquatics, we’ll walk you through how to choose the right fish tank size for your home, how to calculate the space you’ll need, and how your setup — freshwater or tropical — affects your choice.

Step 1: Assess the Space You Have Available

Before buying an aquarium, it’s essential to consider where it will live in your home. The placement and available space will directly impact the tank size you can comfortably accommodate.

Considerations for Placement

• Surface strength: A filled aquarium is heavy. Water weighs roughly 1 kg per litre, so even a 60 litre tank will weigh over 60 kg once you add gravel, decorations and equipment. Make sure your stand or furniture can safely support it.
• Accessibility: You’ll need space around the tank for feeding, cleaning, and equipment maintenance. Leave room for filter access and wiring.
• Light and temperature: Avoid placing the aquarium in direct sunlight (which can cause algae growth) or near heat sources like radiators, as this can make temperature control difficult.
• Visibility: Choose a location where you can enjoy watching your fish without too much foot traffic nearby — sudden vibrations or movement can stress them.

Once you’ve chosen the best spot, measure the maximum length, depth, and height available. This will help you determine the tank size range you can consider.

Step 2: Understand How Tank Size Affects Fish and Maintenance

A common beginner mistake is choosing a tank that’s too small. Smaller tanks might seem easier to manage, but they’re actually more difficult to keep stable because water conditions can fluctuate quickly.

Benefits of a Larger Tank

• Easier maintenance: Larger volumes of water dilute toxins such as ammonia and nitrites, keeping conditions more stable.
• Happier, healthier fish: Fish need room to swim and establish territory. Overcrowding leads to stress, aggression, and illness.
• More options for fish: With a bigger tank, you can keep a wider variety of species and more complex aquascapes.

The Rule of Thumb

For beginners, a good rule is 1 inch of fish per gallon (or roughly 2.5 cm per 4.5 litres) of water. However, this is only a general guide — it doesn’t account for species with higher space needs, such as cichlids or goldfish, or for equipment and décor taking up room.

Step 3: Match Tank Size to the Type and Number of Fish

The size and behaviour of your chosen fish will determine how large your aquarium needs to be.

• Small community fish (like guppies, tetras, and rasboras): 60–100 litre tanks work well for small groups.
• Medium fish (like angelfish, mollies, or dwarf gouramis): A 100–150 litre tank allows enough swimming space.
• Larger or territorial fish (like cichlids or goldfish): You’ll need 200 litres or more, depending on the number of fish.

Always research the adult size and temperament of each species before buying. Juveniles may look small now, but they’ll need much more room as they grow.

Step 4: Freshwater vs Tropical Aquariums — How It Affects Tank Size

The type of aquarium setup you choose also influences how large your tank should be.

Coldwater (Freshwater) Tanks

These are ideal for beginners and usually home to hardy fish such as goldfish or minnows.

• Pros: No heater required; generally easier to maintain.
• Cons: Fewer species options; some fish grow quite large.
• Tank size tip: Goldfish in particular need much more room than many people realise — ideally 100 litres for the first fish and 40–50 litres for each additional one.

Tropical Tanks

Tropical setups allow you to keep a wide range of colourful fish, but they require more equipment and stable water conditions.

• Pros: Greater variety of species and plants; vibrant aquascapes possible.
• Cons: Requires heater, thermometer, and often more filtration.
• Tank size tip: Because of the added equipment, you’ll need to factor in space for heaters, internal or external filters, and lighting systems. A 100 litre tank is a good starting size for most tropical beginners, as it offers stable conditions for small community species.

Step 5: Factor in Equipment and Maintenance Space

Even if your measurements suggest a certain tank size fits, remember that equipment takes up space both inside and outside the aquarium.

Equipment to Account For:

• Filter: Internal filters sit inside the tank and reduce swim space. External canister filters need space beside or underneath the aquarium.
• Heater and thermometer: Necessary for tropical tanks — allow room for these to be fully submerged and easily accessible.
• Lighting: Some tanks have built-in lighting, but if you’re adding separate units, check they fit within the space above the tank.
• Air pumps and tubing: These often sit outside the tank but still need nearby access.

Also think about maintenance tasks like water changes — make sure there’s enough room to siphon water, remove decorations, and clean without difficulty.

Step 6: Choosing the Right Tank Shape and Design

Tanks come in a variety of shapes — rectangular, cube, bow-front, or even column designs — and the shape you choose affects how much usable space your fish actually have.

• Rectangular tanks provide the best surface area for oxygen exchange and plenty of horizontal swimming space.
• Tall tanks may look stylish but can limit the number of fish because of less surface area.
• Nano or cube tanks are suitable for small species or single betta fish but require diligent water monitoring.

Step 7: Start with a Practical Plan

Once you’ve assessed your space, considered your fish choices, and accounted for equipment, you’ll have a realistic idea of what size tank will work best.

For beginners, a 60–100 litre aquarium is often the perfect balance — large enough to be stable and forgiving, but still compact enough for most living spaces.

Final Thoughts

Choosing the right aquarium size is about more than aesthetics — it’s about creating a stable, healthy environment for your fish to thrive. By considering your available space, the equipment you’ll need, and the type of fish you plan to keep, you can make an informed choice that will make fishkeeping both enjoyable and sustainable.

Start with the right foundation, and your aquarium will not only be a beautiful feature in your home but also a long-lasting, thriving ecosystem.