The 'rule'-
1 inch of fish per 1 gallon of water in the aquarium
Some people say for UK it is 1cm per litre, however this doesn't correlate with the US figures. 1cm per litre would come out nearly twice the stocking density of the US rule.
Important Conversions-
1 inch is approximately 2.5 centimetres
1 gallon is approximately 4.5 litres
Sadly, this rule of thumb is commonplace on the internet and even some pet shops will advise it. I would even say it is responsible for the majority of fish 'abuse' due to how often it is referred to. A simple search will bring up countless articles of 'How many fish can I keep in my tank', with most answers following this rule. Also- despite popular belief- fish DO NOT grow to the size of their tanks.
Three of the most popular aquarium fish are Neon Tetra, Goldfish and Betta (otherwise known as a Siamese Fighting Fish). Neon Tetra, on average, grow to about 2.5 inches, betta no more than 1 inch and the average Comet Goldfish maxes out at around 12 inches!! According to the highly flawed equation, a male betta could happily live out its shortened lifespan in a tiny 1 gallon bowl. For you to imagine, this is about the same as the volume of a large mixing bowl. Could you ever think of a fish living in something that small? The very minimum tank size for a betta is 5 gallons, so this proves the equation wrong. It often leads to large, active fish in tiny tanks and even big aquariums are often over stocked. The inch of fish per gallon rule has long been the discussion of fish keepers since it was thought up: in this blog we will be looking at why.
Firstly, there are many factors that the equation does not account for. This includes the activity of the fish, the social needs of fish and also their bio load, amongst others. The most important of these is how active the fish species is. Using the popular Neon Tetra as an example, some fish are very busy and keep moving, therefore requiring a larger swimming space. 10 of these fish would need a 20 gallon tank to live when taking into account the inch per gallon rule. Whilst many sources state this is enough, the additions of gravel, rocks, hides and other decor pieces quickly swallows up valuable volume and swimming space. A 10 gallon tank won't hold 10 gallons of water! With this in mind, I would try to get a larger tank. Comet Goldfish, another popular and highly active species, can reach 12 inches, so a tank of just 12 to 15 gallons would be prefect, right? Wrong! A 15 gallon tank is about 2ft (60cm) long, so this fish could become half of the size of its tank's length, leaving it unable to even turn itself around. The actual tank size is a minimum of 30 gallons, however bigger is always better. In my opinion, goldfish, shubunkins and other similar large species do best in good sized ponds, which is a more natural environment. Betta are commonly seen as vase or bowl fish, despite being very active. This means- as a result of terrible internet sources, social media and improper research- they are the most commonly 'abused' captive fish.
Another issue with the equation is that it forgets the social needs of a fish (loosely covered above). Some species, such as our Neon tetra example, are schooling fish. This means that they live in groups. They are very social, and feel safer in large groups (safety in numbers) but they also look better in the aquarium when a group all swim around together. Most tetra species are the same and should be kept in group of at least 6. The equation tells us that we can but one neon tetra in a 2.5 gallon tank. Now we know already that because of activity levels and the need for companionship, this is not true. As mentioned above, the rule would equate to 10 fish in a 20 gallon tank, which some say is ideal. I would say to take information regarding tank sizes from any source with a grain of salt, and just always go for the biggest tank you can get, planning stock accordingly and responsibly. One the other hand, male betta must be kept alone (some females can coexist with caution) but the size recommendation of 1 inch is still way off the needs for this beautiful species.
Bio load (waste from the fish) is one of the most important factors in tank planning as it determines not only your tank size and stock, but also the filtration. If a small tank, with a filter designed to work with the same sized aquarium, hold a fish to large or too many fish, ammonia and other killers will begin to build up in the tank quickly, killing the fish within a matter of days. This is the most common mistake in most aquariums, and why I always over filter and under stock my aquariums. Other aquatic animals, such as turtles, should also have over filtered aquariums as they have a very high bio load.
Did you know that fish aren't like pencils? They grow taller and wider too! The creator of this rule clearly didn't get the message as it only account for the length of the fish. Some species of fish don't get too long, but instead grow very tall or bulky, such as discus. This is a huge flaw, which can leave many species with little swimming space, particularly if there is a lot of hardscape decor.
One other common misconception is what the size of the fish will end up being. People see tiny 1cm fish in the shop, unaware that it could turn into a 1ft beast in just a few years. When using the inch per gallon theory, possible new owners who have not done proper (if any) research into the species and its requirements could buy 20 of these for their 10 gallon tank. Unfortunately, most pet shops both in the UK and other countries will not screen buyers, allowing this to occur.
In conclusion, the inch per gallon rule of thumb forgets many key aspects of fish keeping, which therefore renders it practically useless. Could you ever imagine a human equivalent to this? I wouldn't like to...