Starting an aquarium is an exciting step into a fascinating hobby—but one of the most common beginner mistakes happens before the first drop of water is added: choosing a tank that’s too small. While compact aquariums may look cheaper, easier, and more convenient, the truth is that larger tanks are far more stable, forgiving, and enjoyable, especially for first-time fish keepers.
This guide will walk you through the main aquarium types, the equipment each requires, realistic size recommendations, and the types and numbers of fish they can comfortably house. By the end, you’ll understand why—when it comes to aquariums—bigger really is better.
Of course, it’s always important to take expert advice too and, whilst this guide can help to point you in the right direction when making purchasing decisions, we’d also highly recommend speaking to aquatics experts, such as those at Complete Aquatics, to get guidance tailored to your particular circumstances.
Why Tank Size Matters More Than Anything Else
Fish live in a closed system. The smaller the volume of water:
- The faster toxins (like ammonia) build up
- The quicker temperature and water chemistry change
- The harder it is to correct mistakes
Larger tanks dilute waste, buffer temperature swings, and give fish more room to behave naturally. For beginners, this means fewer problems, healthier fish, and less stress—for you and your livestock.
- Coldwater Aquariums
Typical Fish
- Goldfish (common, comet, fancy varieties)
- White cloud mountain minnows
- Danios
Equipment Needed
- Tank (glass or acrylic)
- Powerful filter (goldfish produce a lot of waste)
- Air pump & airstone (optional but helpful)
- Lighting (basic, mainly for viewing)
- No heater (most coldwater fish prefer 18–22°C)
Recommended Tank Sizes
- Minimum for beginners: 75–100 litres (20–26 gallons)
- Single fancy goldfish: 75 litres minimum
- Each additional goldfish: +40–50 litres
Bowls and tiny tanks are never suitable for goldfish.
Stocking Ideas
- 2 fancy goldfish in a 120-litre tank
- 10–12 white cloud minnows in a 60–75-litre tank
- Tropical Freshwater Aquariums
This is the most popular choice for beginners due to the wide range of colourful, hardy fish available.
Typical Fish
- Guppies, platies, mollies
- Tetras and rasboras
- Corydoras catfish
- Dwarf gouramis
Equipment Needed
- Tank
- Filter (internal or external)
- Heater (typically set to 24–26°C)
- Thermometer
- Lighting (standard LED is sufficient)
- Lid or cover (many tropical fish jump)
Recommended Tank Sizes
- Absolute beginner minimum: 60 litres (15 gallons)
- Ideal beginner size: 100–125 litres (26–33 gallons)
Smaller tropical tanks can work, but they leave very little room for error.
Stocking Ideas (100–125 litres)
- 10–12 small schooling fish (e.g. tetras)
- 6 bottom-dwellers (e.g. Corydoras)
- 1 centrepiece fish (e.g. gourami)
- Planted Freshwater Aquariums
Planted tanks combine fishkeeping with aquascaping and require a bit more planning—but they’re incredibly rewarding.
Equipment Needed
- Tank
- High-quality lighting (plants need it to thrive)
- Filter (gentle flow preferred)
- Heater (most plants are tropical)
- Nutrient-rich substrate
- Optional: CO₂ system (for advanced setups)
Recommended Tank Sizes
- Beginner planted tank: 75–100 litres
- Larger tanks make lighting balance and plant growth much easier
Stocking Ideas
- Small schooling fish (rasboras, tetras)
- Shrimp and snails
- Peaceful bottom-dwellers
Planted tanks often support slightly higher stocking, as plants help absorb waste—but moderation is still key.
- Marine (Saltwater) Aquariums
Marine tanks are stunning but not ideal for first-time aquarists unless you’re committed to learning.
Equipment Needed
- Tank
- Marine-grade filtration
- Protein skimmer
- Heater
- Powerheads for water movement
- Marine lighting (essential for corals)
- Refractometer or hydrometer
- RO/DI water system (recommended)
Recommended Tank Sizes
- Minimum beginner marine tank: 125–150 litres
- Larger volumes are far more stable in saltwater systems
Stocking Ideas
- 2–3 small marine fish in a 150-litre tank
- Gradual stocking over several months
- Corals only once the tank is mature
Choosing the Right Tank Shape
- Long tanks = better swimming space and oxygen exchange
- Tall tanks = look impressive but are harder to light and maintain
- Wide tanks = ideal for bottom-dwelling fish
When in doubt, choose length over height.
Handy Tips for First-Time Aquarium Buyers
- Buy the biggest tank you can realistically afford and accommodate
- Avoid tanks under 60 litres unless you’re experienced
- Research fish before buying the tank—not after
- Don’t rely on “1 inch of fish per gallon” rules (they’re outdated)
- Check adult fish size, not shop size
- Leave room for filtration and equipment
- Bigger tanks are easier to maintain than small ones
- Plan for growth—fish outgrow tanks faster than you expect
- Stability matters more than simplicity
- Take your time—rushing leads to losses
Final Thoughts
Choosing the right aquarium size isn’t about excess—it’s about creating a stable, healthy environment where fish can thrive. While small tanks may seem beginner-friendly, they demand near-perfect maintenance. Larger aquariums give you wiggle room, better water quality, and a far more enjoyable experience.
When starting out, remember this golden rule:
You’ll never regret buying a tank that’s too big—but you’ll almost always regret buying one that’s too small.
