How to Choose an Aquarium Tank Size – Ultimate Guide

Many considerations weigh in when deciding on an aquarium size. 

Traditionally, the volume of fish you intend to stock in the tank (per square inch) gives you the tank size you need. For every inch of fish body, you require a gallon of tank space – but this is far from perfect when choosing a tank size. 

There are many other valuable considerations before choosing a tank size. This guide will help you make the best (and most informed) decision.

The Traditional Consideration – Fish Size

The most basic way to calculate the tank size is to determine the combined fish size per inch (for your adult fish, excluding the fins). 

So, if you want to stock 10 fish with a combined “fish space” of 10 inches, you must get at least a 10-gallon tank. However, this simplistic measure isn’t sufficient to ensure your fish have enough room to move and school, and it certainly doesn’t give you space for decorations, hardware, and more. 

In short, sticking 10 fish of one inch each in a 10-gallon tank is a bad idea as your fish won’t thrive, and the tank will quickly be overcrowded.

A Comprehensive Overview of Tank Size Considerations

Tanks can be costly, especially if you want to create a complex aquatic environment with different fish species and don’t want to make mistakes. These are the fundamental considerations to help you make the perfect tank size decision.  

1. The Species of Fish

What species of fish you intend to stock in your tank is the most important deciding factor for the tank size. Fish that are fast-moving and prefer to move in schools will require a larger and more vertically spacious tank. If your fish species are territorial, you must also ensure enough space to prevent fish aggression. 

Should your tank be communal, you must accommodate each species’ preferred water depth and space needs. You certainly don’t want your tank to end up being a blender filled with fish all chomping on each other!

You can think of your tank as being in zones, with a zone for each fish species, affording them enough space for their natural behavior and needs. 

2. Tank Decorations and Aquatic Plants

Next, your tank is unlikely to be entirely bare, so you want to factor in what plants, decorations, or aquascaping you intend to add. Your fish will have specific plant needs, so if you are stocking larger fish with smaller fish, your smaller fish will prefer plenty of aquatic forests to hide in and spawn their eggs. 

While your fish will love the plants, these also take up space, and you should add that to the tank size considerations. If your total mass of plants (once the plants have matured) equals 5 gallons, you should add that 5 gallons to the overall tank size.  

3. Tank Hardware and Sump Chambers

Okay, your tank will need cleaning and maintenance, so you have to allow space for the hardware your tank requires. If the filter, tubing, heater, thermometer, and any other hardware you need, such as air stones, submersible lights, and wave makers, can all fit inside a 3-gallon milk jug, then you have to factor in the extra space to your tank size. 

Where you would have gotten a 20-gallon tank, you’d now require a 23-gallon tank to ensure the hardware doesn’t cheat your fish out of space. 

If you intend to hide your hardware with a sump or hidden filtration chamber, you should also factor this into your overall tank calculations. Some manufacturers helpfully indicate the overall tank size and the size of the sump or filter chamber, letting you know exactly how much space is available to the fish. 

4. The Unforeseen Adds

Unless you can predict the future, you probably don’t precisely know what you will add to your tank, especially in a few months. You may find a gorgeous aquatic plant you’d like to include in your tank, but it requires some extra space. Planning for the different and unintended adds is the best way to ensure your tank isn’t crowded.

Adding an extra 2 – 3 gallons of tank space for the “what if” in life you can’t really plan for is great.

5. Budget and Costs

Finally, the most significant consideration is the budget you have at your disposal. Purchasing a large tank and then magically setting it up is never a simple matter. You must ensure the tank (and all of its accessories) are within the amount you want to spend. 

There’s no point in buying a large tank and being unable to afford the monthly maintenance costs, the price of the necessary hardware, and operational expenses (like an increase in your utilities bill). 

Also Read: 10 Best Filters For A 55 Gallon Fish Tank

Tank Sizes to Consider

Aquarium Tank Size

Manufacturers often design a range of tanks, but some sizes are commonly used for different functions, such as the popular Betta tanks and racing tanks of Neon Tetras. Here are a few widely used tank sizes: 

  • 1-gallon to 15-gallon tanks: These are suitable for smaller size fish due to the limited space. With a weight of up to 170 pounds once filled, these tanks aren’t for beginners. 
  • 15-gallon to 40-gallon tanks: These are considered medium-sized tanks and great for community tanks with larger fish species like angelfish and clownfish. You can also use these tanks for breeding purposes.
  • 50-gallon to 150-gallon tanks: When you’re ready for a large tank, you can consider a 50 – 150-gallon tank. You can stock these tanks with multiple fish species, cultivate corals, create a complex reef structure, or build a magnificent planted aquascape. 

There are many different types of tanks you can consider, and what you choose will depend on your unique tank needs. However, we’d like to discuss some of the basic tank types available and offer an example of each.

Show Tanks

Show tanks are chosen for the viewing quality they offer. You would look for a tank with a large front pane, and usually, these tanks are made from low iron glass with superior clarity. 

The type of show tank you’d get depends on the abovementioned factors. If you are looking for a show tank for your coral collection, you’d go for one with lower sides and a larger bottom panel to have a large surface to accommodate your corals. Shallow water also means better growth for the corals and lets you see them in greater detail. 

Likewise, if you want a tank to showcase your Neon Tetras that race around, you’d go for a shallow but long tank to give them enough space to lap each other. 

Show tanks tend to be larger, giving you enough space to really showcase your aquatic collection. 

Fzone 17 Gal Rimless Low Iron Fish Tank

Fzone 17 Gal Rimless Low Iron Fish Tank

A show tank can be any size, from a 5-gallon tank to a 150-gallon tank. What is important is the quality of the design and the viewing clarity, and that’s what the Fzone 17-gallon low iron fish tank offers. This quality tank is made with low iron-containing glass, providing a more precise view thanks to the improved transparency and eliminating color tinting. 

With the beveled 45° angles, the tank presents a seamless view, with as little distortion in the corners as possible. While the tank doesn’t include a filtration chamber, you can easily install one to hide your tank hardware.  


  • Made from high-quality, low-iron glass
  • Excellent finish with beveled and mitered edges for joints
  • Includes pre-installed black foam leveling mat to ensure perfect placement
  • 6 mm glass thickness for strength
  • The edges are precision sealed with silicon
  • Tank is quality tested with water for 72 hours before shipment


  • The tank is quite pricey, and you’d still have to purchase your tank equipment 

Also Read: Best Aquarium Guide: Choosing The Best Tank

Breeding Tanks

Each tank has its own purpose, and if you are into breeding, you may have unique requirements to ensure your new generation of fish survive and thrive. The primary consideration for a breeding tank is that it is the correct width or length to allow breeding dividers to be installed when your fish are ready to spawn or for the fries to hatch. 

Breeding tanks are less concerned with the quality of the view and more with the functionality to ensure your fish offspring survive. Ensure you choose a tank with glass thickness that allows for your preferred breeding box to fit over the rim.

Fluval Multi-Chamber Holding and Breeding Box

Fluval Multi-Chamber Holding and Breeding Box

Our choice here is not for a tank but rather the breeding box that can sit inside the tank or over the outside. This option by Fluval is excellent for any avid fish enthusiast. You can use it for your fish to spawn or to isolate sick fish. 

Simply slide the box over the tank rim, letting the box hang on the outside of your tank. An aerator’s bubble function allows water to enter and cycle through the box for oxygenation and to clean out fish wastes.

We include this option in our tank selection as it influences your choice in breeding tanks. This breeding box is 10.25 inches long, so your tank must be at least 10.25 inches wide or long to accommodate the box. We think the Lifegard Aquatics 5 Gallon Rimless Clear Glass Aquarium or the Lifegard Aquatics 10 Gallon Rimless Clear Glass Aquarium are great choices.


  • Three chambers, which are perfect for isolation or breeding uses
  • Made from quality acrylic for unobstructed views into the breeding box
  • It is affordably priced and comes in two sizes


  • The water from the breeding box is recycled into the tank, which can spread disease

Regular or Hobby Tanks

For the hobbyist, the overall design of the tank may be a small consideration as they are likely on a strict budget. Starter kits are ideal as these contain the tank and any hardware required. 

Ensure you choose a hobby tank with enough space for the hardware and the fish you’d like to keep in the tank. Manufacturers sometimes include hardware that’s large and in the way, so choose carefully. 

Koller Products Plastic 3-Gallon Aquarium Starter Kit

Koller Products Plastic 3-Gallon Aquarium Starter Kit

If you’re a hobbyist and just starting in aquariums, you’d do well with a starter kit like the Koller Products plastic 3-gallon aquarium. While plastic or acrylic tanks often get a bad reputation, this is an ideal option for a newbie as it’s lightweight, easy to maintain, and offers seamless views. 

Setting up this tank is super easy, and the molded plastic tank includes a convenient cubicle for the filter, which helps you maximize all your tank space for your fish and decorations. The domed lid includes an inbuilt LED light so you can start your aquatic hobby. This tank is perfect for a few Bettas, shrimp, and some aquatic plants. 


  • Excellent price 
  • Includes everything to start the tank
  • Made from impact-resistant acrylic
  • Seamless views


  • The tank lid is a bit bulky 

Specialty Tanks

Other tank types to evaluate include round tanks with curved fronts, fish bowls, and shaped or self-contained tanks. Your choice will depend on what you want to place in the tank. The dimensions of your tank will depend on what purpose it serves, such as decorative or specific fish species.

PRUGNA Wall-Hanging Fish Bowl 

PRUGNA Wall-Hanging Fish Bowl

The wall-hanging bowl is ideal for a study or entrance hall as you simply attach the domed bowl to a wall for an instant feature. The bowl fits vertically on a wall, with an opening to add water and fish and feed your fish. The real selling point of a wall-hanging bowl is that it doesn’t require counter or table space as the bowl is mounted on the wall. 

The bowl can also function as an aquatic plant planter where plants can root and establish before being transferred to a larger tank.

Your space requirements are pretty minimal with a wall-hanging bowl, and specialty tanks are often less than a gallon in size. 


  • Ideal for a single betta fish
  • Highly decorative element for a statement feature
  • Lightweight
  • One gallon in size
  • Versatile “tank”


  • Cleaning can be challenging as you have to replace water and siphon the substrate frequently

Also Read: 7 Best 30 Gallon Fish Tank Kit

Final Tank Thoughts

Your aquarium needs are unique, and your tank has to meet these. Deciding on the size tank to get depends on what you intend to place in the tank. Again, you should consider various factors before just rushing out to buy the first good-looking tank. Here is a little checklist to help you shop:

  • Consider your fish species for size, behavior patterns, and hardware needs
  • Factor in your tank decorations’ overall space requirements and add to the tank size
  • Add up the space required for hardware and add this to the fish space requirements
  • Add in a little extra room for future changes
  • Your budget and maintenance costs 

With this information, you can shop for your ideal tank or simply click on one of our reviewed tanks to meet your needs. Happy shopping!

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