Many people starting the hobby are mis-informed about exactly how many and what type of fish they can keep in a tank. For example, I've seen someone at a local fish store recently asking about why their chinese algae eater was growing so big and attacking other fish in their five gallon tank. I have also seen instances on other forums where beginners asking questions are mis-informed and say it's alright to keep red-tailed black sharks in a ten gallon, because they have had success with it and all was well.
In addition to giving away fish not suitable for the size of a tank, local fish stores also use the "inch per gallon" rule. Basically, for every inch of fish, you must have one gallon of water for. So, for example, if you kept six neon tetras, their max size is one inch. That is six neon tetras. Thus, you will need six gallons of water because that is six inches of fish. So basically, a two-foot pleco would need twenty-four gallons of water, a bala shark would need twelve gallons of water, a dwarf gourami would need two to three gallons of water, and so on and so forth.
This rule has many faults. Here is a a quote from a book I got at the library a year ago (The Complete Fishkeeper, by Joseph S. Levine) and have been renewing and checking it out ever since. Although it's inaccurate on some topics and outdated, it's much more useful than other publications. Contrary to most, it explains the nitrogen cycle in-depth, why small tanks aren't good habitats for most fish, why saltwater is not something to start off with, and most importantly, why the inch per gallon rule does not work.
Here is the section that explains the inch-per-gallon rule:
" 'How many fishes can I keep in my aquarium?' is the most common beginners' question. many books recommend 'one inch of fish per gallon of water without aeration, two inches of fish per gallon of water with aeration.' That formula has caused more grief to more fishes than any other bit of misinformation. In truth, such formulae all fall short, because stocking density depends on sizes and personalities of fishes, filters used, tank shape, and water temperature.
"Furthermore, you should realize that there are two ways to approach this question. Usually, the beginners are really asking, 'How many fishes can I stuff in my tank without them all turning belly-up in the morning?' It makes more sense to ask, 'How many fishes can coexist comfortably in my tank, in a way that allows me to keep things running with minimal effort?'
"The answer is not necessarily straightforward. No matter how large your tank, it is infinitesimally small compared to natural habitats. On a reef or in a pond, individuals of aggressive species have plenty of room in which to avoid one another once a contest between them has been decided. But even a fifty-five gallon tank-which most of us think of as quite large-offers little refuge for the vanquished, who can be continually harassed by the victor. And even generally peaceable fishes may harass one another if crowded too small in a space.
"For these reasons, it is particularly important to observe warnings about species that do not get along with others of their own kind. Nearly everyone knows that male Siamese fighting fish won't tolerate one another, but there are many other, less flamboyant species that fight just as intensely. In nature, their aggressive behavior spaces them out with many yards between them; in tanks, without room to roam, they fight both more frequently and more intensely.
" There are also some fishes-such as the giant danios (Danio malabaricus)-that are peaceable but very active swimmers. Such species fare well in ten-gallon when small, but need much more room to swim when three or four inches long.
"That's why experienced aquarists stock tanks more by intuition than by calculation. Two dozen large cardinal tetras in a twenty-gallon tank, for example, present an entirely different-and far more peaceful-situation than two dozen half-grown cichlids of the same size."
What I thought was important I wrote in red. Although it is a bit inaccurate, for the most part it tells you why this rule does not work.
Let's go back to the common pleco. According to the inch per gallon rule, this fish needs twenty four gallons of water. However, that is not a suitable environment for this fish at all. Even if the tank was twenty four inches long, it is still not suitable for this fish because a) it has a huge bioload that can easily overload most filters in that small of a tank, and b) in that small of a tank it can easily stunt itself, which is a process where the fish do not grow to their potential size, and their organs are squished together. It is a highly painful process for fish.
Let's also go back to the neon tetras. Although small, these fish are very active. Putting them in six gallons of water will do you no good, because these fish need much more swim room than that. A ten gallon is far better for these fish than a six gallon.
Make of this what you will. You can either continue to use the inch per gallon rule, stop using it if you are, or if you have already stopped keep informing people of why it is important to be responsible and choose fish wisely. No common pleco-no matter how bad of algae eaters and how messy they are-deserves to be stunted in a ten gallon tank because the aquarist keeping them was misinformed on the "inch per gallon" rule.
Keeping fish alive requires putting them in water, but keeping them happy and healthy requires research, effort and responsibility. Choose wisely.
