How to pick an aquarium size?
Most fish need space, and the more they have the better they tend to get along. When fish are crowded they become more agitated and are more likely to quarrel with tank mates. A general rule of thumb for stocking a fish tank is one inch of adult size fish per net gallon of aquarium capacity, but territorial fish need even more space. Remember that the fish you buy will probably grow, and a 100-liter aquarium doesn’t actually hold 100 liters of water when you factor in internal dimensions, gravel, and decorations. Also, what to us is a large aquarium (750+ liters), is still just a fraction of the space fish have in their natural habitats.
Keep in mind that different fish prefer different shapes and swimming spaces. Wider aquariums give active fish, like danios and barbs, the space to spread out, which in turn helps them get along better. On the other hand, tall, narrow aquariums are appealing to look at and fit into narrow spaces but don’t offer fish as much swimming space or territory as a wider aquarium. These aquariums should be used for less active fish like discus, angelfish, and gouramis.
What are the sizes you should consider?
Up to 30 Liters: Professionals recommend 18-liter tanks as the minimum aquarium size for just one fish. Tanks under 18 liters (like fishbowls) are extremely susceptible to fluctuations in pH and buildups of harmful chemicals and waste materials. They also lack the surface area for healthy gas exchange and unfortunately are stressfully small for even one lone fish; without enough room to swim, a fish can become distressed and more vulnerable to disease. Small fish tanks aren't completely useless, however! They can be vibrant homes for beautiful aquatic plant life and algae, including trendy marimo moss balls (gorgeous, fluffy balls of vivid algae that can live for decades). Tanks that hold up to 10 gallons still have their challenges; due to their small capacity, good filtration and diligent pH monitoring will be necessary to ensure that the environment doesn't become too unstable for your fish.
40 to 70 Liters: Tanks that hold between 40 to 70 liters are very affordable and adequate for fish owners looking for manageable tanks that won't take up too much space. Aquariums at the lower end of this range tend to be decorative and are used as accessories to your home decor rather than centerpieces. That said, even 70-liter tanks are considered small by experts and can only comfortably house a handful of smaller fish. These tanks will still require vigilant maintenance to ensure the proper filtration and chemical balance. If you're a beginner looking for aquarium sizes to hold approximately a half dozen fish under 3" in length, a tank that holds 40- 60 liters might be the one for you.
70 to 150 Liters: Aquariums with capacities between 70–150 liters are optimal for keeping approximately half a dozen to a dozen small fish healthy and happy. Their volume ensures that the water quality isn't overly temperamental, and thus they are a bit more forgiving to less vigilant maintenance. While you will pay more for these larger, heavier tanks, their size is ultimately rewarding. Once you have your tank set up with its substrate and environmental features, your aquatic world, and its fish inhabitants will be the focal point of any space.
Over 150 Liters: Aquariums that hold 150 liters or more (some aquariums hold over 1000 liters!) are optimal for beautifully and comfortably displaying a diverse range of fish. Big fish tanks are stunning in appearance and make lovely homes for fish of many sizes. That said, they come with their fair share of challenges. Large fish tanks, especially tanks that breach 340 liters in capacity, are extremely heavy and can be hard work to keep clean. Their weight alone may require you to purchase a special aquarium stand or install structural reinforcements in your home's floors to prevent damage. It is helpful to note that many large fish tanks come with their own built-in stands, but this means you must keep in mind the dimensions and weight of the whole unit when determining whether it will fit in your space.
Will fish only grow relative to the aquarium size?
This is a misconception that is not true. Larger fish will soon need a larger aquarium to be healthy and happy. Restricting them to a smaller aquarium may stunt the fish's growth and result in physical deformities. Be sure to give the right environment to each species of fish in the aquarium.