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13.1. Stocking Opinions

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Author : David Bogert

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Time To Read :
11 minutes
Difficulty : Level 6

Excerpt :

The topic of stocking is largely OPINIONS, not experimentally derived scientific facts.
Stocking is a very controversial topic. People on social media will adamantly argue that keeping a single betta in anything less than a 5 gallon (20 liter) is incredibly cruel. Many say Lake Malawi Cichlids can’t be kept in anything less than a 50-gallon (200 liter) tank and even that should only have a maximum of ten fish. And the idea of keeping ANY goldfish in less than a 30-gallon (120 liter) tank sends some people into absolute fits of hysteria and rage.

The topic of stocking is largely OPINIONS, not experimentally derived scientific facts. Let me repeat that:

The Topic of Stocking is Largely OPINIONS

In MY OPINION, if one has excellent filtration, with a large volume of good media, in an aquarium established for at least four months, without filter cleaning, the number of fish one can put in a tank is huge. This is NOT “cruel” in any way to fish, in MY OPINION.

The basic stocking “rule” (actually only an OPINION) that has prevailed forever is “one inch of fish for every gallon of water” (one centimeter of fish for every liter of water) . This is a decent VERY gross approximation for beginner’s with their first aquarium and fish that are two to three inches long, like platies. The “rule” breaks down rapidly above three inches and for more advanced hobbyists. This article will give the stocking far more accurately by basing the stocking on the weight and metabolism of the fish, not the length.

Heavily Stocked Aquarium
Heavily Stocked Aquarium

Note that it is important to look up the adult size of a fish before making stocking decisions. A clown loach can ultimately be over a foot in length and won’t do well long term in a 29 gallon tank. But also note that this “long term” means one can have something like a clown loach for several years until it gets too big for the aquarium. At that point the fish will need to be “re-homed” or a larger aquarium purchased.

Some of the common OPINIONS parroted around social media as scientific facts about stocking include:

  • “Overstocking” a fish aquarium will shorten the life of a fish and is cruel.
  • A fish can appear healthy and fine but not be “thriving”.
  • Each fish has a minimum aquarium size and length beyond being large enough to swim freely in.
  • Fish in an aquarium which is too small will not grow to their full size (with the notable exception of goldfish).
  • Have one gallon of water for every inch of fish. (one centimeter of fish for every liter of water)

All these OPINIONS are simply not correct in my OPINION.

Heavily Stocked Malawi Aquarium
Heavily Stocked Malawi Aquarium

With very good filtration with huge amounts of very good media which has been in place uncleaned for four months or more, one can stock at the heavy levels shown below. Let us be very clear, we only “recommend” these heavy stocking levels ONLY FOR  WELL FILTERED AQUARIUMS WHICH HAVE LARGE FILTERS WITH GOOD MEDIA THAT HASN’T BEEN FREQUENTLY CLEANED!!!!! Heavy stocking requires ten times more filtration than light stocking and three times more filtration than moderate stocking.

Heavily Stocked Mbuna Aquarium
Heavily Stocked Mbuna Aquarium

With “normal” or moderate filtration that has been in place for one month divide those numbers by three to get the “moderate” levels. With “normal” or moderate filtration in a new tank divide the numbers by ten to get the “light” levels. The numbers are based on volume and weight.

If one wants to see what is meant by heavy stocking and how often it is used quite successfully, just open this link:

17.4.1. Malawi Aquariums

OB Peacock
OB Peacock

The Correct Math of Stocking

While what the numbers mean is just OPINION,  the correct way to derive the numbers is not open to a lot of debate. Scientist have researched that area and come up with the appropriate formulas.

Stocking needs to be looked at from the aspect of volume or weight, which is the cube of the length (the length multiplied by itself twice). It should never be looked at from the concept of length per gallon, for instance the incredibly inaccurate “one inch per gallon” “rule”. (one centimeter per liter)

What this means is that one 2 inch (5 cm) fish is 8 TIMES heavier than one 1 inch (2.5 cm) fish. And this in turn means that eight 1 inch (2.5 cm) fish are the equivalent of one 2 inch (5 cm) fish when it comes to stocking by weight alone. By the same simple calculations, one 8 inch (20 cm) fish is 8 TIMES heavier than a one 4 inch (10 cm) fish. And this in turn means that 8 four inch (10 cm) fish are the equivalent of 1 eight inch (20 cm) fish when it comes to stocking by weight alone.

But the metabolism of the smaller fish can be four times more than the larger fish. So smaller fish can put out four times the waste of larger fish on a gram per gram basis. This consideration has to be calculated into the tables.

eleven goldfish in a 4 gallon
eleven goldfish in a 4 gallon

The charts in the article on “13. Stocking” do make corrections to the formulas to account for something called “metabolic weight”. Small fish tend to be much more active than large fish (up to 4 times more active). So small fish can eat more and produce much greater bioloads per gram than most large fish. So a correction factor needs to be applied.

Otopharynx lithobates Black Orange Dorsal
Otopharynx lithobates – Black Orange Dorsal

Caution About Aeration

There is one caution about heavy stocking. If the power goes out to a heavily stocked aquarium the fish can die from oxygen starvation in as few as three hours. It is always a good idea to have some sort of aeration on a uninterruptible power supply to any heavily stocked aquarium.

Large Fish In a heavily Stocked Aquarium
Large Fish In a VERY heavily Stocked Aquarium

Overstocking compatibility is very dependent on the fish one is talking about. Most tropical aquarium fish react to overstocking with increased levels of stress and cortisol, the stress hormone. This leaves them slightly more vulnerable to pathogens (a University of Oregon study found 14% more mortality in VERY VERY heavily stocked aquariums of zebrafish versus VERY heavily stocked aquariums, this is not a huge increase!).

OB Peacock
OB Peacock

However, It is my OPINION that small aggressive fish like Lake Malawi cichlids (mbuna and peacocks) are different. Even mbuna have yards of space for each fish in Lake Malawi. In an aquarium, even a large aquarium, Lake Malawi cichlids can’t get away from each other. So multiple fish get stressed, very stressed in many cases. I think this stress gets maximized with roughly five to eight fish in a fifty-gallon aquarium.

The more you get above eight fish in a mbuna aquarium, I believe the stress levels go down, not up. This is based on the fact I get less aggression, fewer frayed fins, and fewer fish deaths as the stocking levels go up in my mbuna aquariums. This is my OPINION!

Heavy Fish Stocking of an Aquarium
Heavy Fish Stocking of an Aquarium

As with all rules, there are obvious exceptions. One YouTube video maker had an aquarium full of predator haps spring a leak. He had to put the occupants of the aquarium in a smaller aquarium already stocked with a moderate load of predator haps. The result was a very heavily loaded aquarium where the fish were bumping into one another. He very rapidly found himself with a lot of fish with missing scales and bites on their sides.

This is hardly surprising. These predators haps have teeth that can take bites out of other fish. In an aquarium that is heavily stocked the fish have no way to get out of the way of a biting attack. They were literally “fish in a barrel”. So, of course, bites showed up rapidly. I recommend heavy stockings for mbuna and peacocks, I decidedly do not recommend heavy stockings for predator haps.

Heavily Stocked Mixed Biotope Aquarium
Heavily Stocked Mixed Biotope Aquarium

I’ve successfully heavily stocked African cichlids at a quantity of roughly 50 five-inch fish in a 100-gallon aquarium (with a huge amount of filtration, water changes, and aeration!). I have had up to ten tanks with this stocking of mbuna and haps for some thirty years. I did lose roughly 50% of the fish in three aquariums once because the power went out and aeration stopped. Lesson learned: get a generator.

Note that many ascribe to some arbitrary “minimum” tank size for each size of fish. For instance, saying a six-inch peacock needs at least a 75-gallon tank. I find no such limitation exists. I see no reason not to have peacocks in a 29-gallon tank. This is my OPINION!

And I’ve been very successful with 15 five-inch mbuna stocked in a 29-gallon tank. The fish were healthy and thriving for many years. The tank was over-filtered to the max with a continuous water change system on it.

Heavily stocked aquarium
Heavily stocked aquarium

Social Media Reactions

Also, be aware that there are some people on the various internet forums who vehemently protest whenever someone shows a heavily stocked aquarium. These individuals take a moral offense to anyone stocking at more than the light levels shown above. Note these individuals typically come from hotbeds for PETA such as the United Kingdom, California, and Australia.

The “fish police” or “fish Nazis” claim that such stocking results in fish death and shortened lifespans and thus can be described as “poor husbandry”, “awful”, “ignorant of the science”, “willfully ignorant”, “behind the times”, “irresponsible” and “crazy”. One lady said I was guilty of “cruelty to animals”.

I always message them and ask for references that I can use to educate myself on the deleterious effects of overstocking. Things such as research articles, books, magazine articles, forum articles, YouTube videos, etc. Not one of the fish police has ever responded with anything.

Heavily stocked Aquarium
Heavily stocked Aquarium

There are three aquarium forums and groups (with many members) that have administrators and the moderators all heavily biased against anything except a very lightly stocked aquarium. Note that all three of these aquarium forums have banned me from their pages for saying one can heavily stock aquariums.

The ability of aquariums to have the heavy stocking calculated here is supported by many photos of heavily stocked aquariums taken from Facebook forums and groups. I count photos of 87 very heavily stocked Malawi aquariums in this article

17.4.1. Heavily Stocked Malawi Tanks

A Heavily Stocked Peacock Aquarium
A Heavily Stocked Peacock Aquarium

There was a Facebook post of an established 55-gallon aquarium that had well over 100 four-to-six-inch mbuna in it. It was an amazing aquarium. The water was crystal clear, and the fish were healthy. The hobbyist over-filtered with two huge canister filters, a large sump, and had a huge amount of surface agitation in the aquarium. This “over-filtration” theme keeps coming up again and again in heavily stocked aquariums with crystal clear water.

Note this aquarist had been keeping fish for fifty years and had three Malawi aquariums just like the 55 gallons, all very heavily over-stocked. He was using a drip water change system. In virtually all aspects of the hobby this man was eerily identical to me, except I have had up to ten drips changed, heavily over-stocked and over-filtered aquariums for fifty years.

Heavily Stocked Oscar Aquarium
Heavily Stocked Mixed Oscar Aquarium

Belief Perseverance Effect

Note that we can state with absolute certainty that anyone who believes that heavy stocking is “cruel” and reads this article will absorb absolutely nothing from this article.

A quote is appropriate. This is what Tumbleweedtumbling wrote in reply to one of the hysterical, “that is incredibly cruel to the fish” comments:

Your response is precisely the kind of hysterical biased personal opinion that my entire comment was about. You have no research to quote, no personal experiments and nothing to offer but your opinion.  I advise you to go back and read what I wrote very carefully. Obviously you missed everything but your personal beliefs being challenged and you are unable to process that challenge or consider you just might be wrong.”

This refusal to understand or accept that one might be wrong is something called “belief perseverance effect”. It says that if someone believes fervently in something, a normal person will rationalize and support their own decision no-matter-what. There is no point in trying to change such a person’s mind, especially when dealing with a topic like what the humane treatment of a fish is.

“Nothing dies harder than a lie that people want to believe”


This is just the world we live in and there is no point in trying to fight it. Every time this article is posted in social media someone comes on to say how horrible this article is and that the author is an idiot.

Incredibly Heavy Stocking of an Aquarium
Incredibly Heavy Stocking of an Aquarium

Stocking in Greater Depth

Hopefully the following links can put some common sense into this subject:

13. Stocking

13.2. Calculating Stocking Ratio

13.3. Lifespan of the Fish

And the common or narrow bodied goldfish should be stocked as a five inch fish.

This is delved into in this link:

17.5. Goldfish