How Many Fish? Beware of the Rule

How many fish can your fish tank hold comfortably?  It’s the age old question that every aquarist wants a quick answer to.  Most new aquarist just want a rule of thumb.  If you have been in the hobby you want to make sure you are not overcrowding your tank if you have had problems.  The golden rule used to answer this question most often might be bad advice if it’s not completely understood.  Because of this, let me suggest a better way.

One Inch per Gallon of Water – The Golden Rule

The most common advice that you will hear for stocking your fish tank is to add no more than 1 inch of fish per gallon of water to your fish tank.  The problem is that the answer to this question is not really as simple as this rule by itself.  There are many other factors that influence whether or not this rule really works.

To help illustrate the point, think of a 12 inch Oscar as opposed 8, 1½ inch Neon Tetras.  They both add up to 12 inches, but the room they actually take up in your tank would be dramatically different.  This shape difference also has a big effect on the waste output of your fish.

For smaller fish species, this rule works pretty well.  For larger, it does not. Also, don’t forget that the rule is referring to the full adult size of your fish.

Then It Must Be Surface Area Rule

The surface area calculation is pretty simple as well although it’s not perfect either.  Basically you multiply the length times the width of your tank.  This gives you your tanks surface area.  Under this rule your tank would house 1 inch of skinny fish for every 12 inches of surface area.  Your tank would also house 1 inch of full bodied fish for every 20 inches of surface area.

You might have already figured out that the surface area calculation does not take into account how tall your tank is.  That is because it us based on the amount of oxygen exchanged.  This only happens on the surface of your tank of course.

So What Do You do?

I would suggest using the 1 inch per gallon rule with one modification.  For smaller fish, use the 1 inch per gallon calculation as it is.  For larger, fuller bodied fish, you’ll want to use a multiplier of 3 and count 1 inch of fish for every 3 gallons of water.  For medium size fish use a multiplier in between like 1 inch of fish per 2 gallons of water.  With this method you can take into account all sizes and shapes of fish.

The oxygen exchange consideration in the surface area method is not that important for the most commonly sold fish tanks.  Most fish tanks sold retail come in pretty standard shapes.  Generally, they will have adequate oxygen exchange whether you have a tall show tank or a longer standard tank shape.  You should however, consider the surface area rule if you do by chance have a very odd shaped fish tank with very little surface area.

What experience have you had with choosing how many fish?

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30 Responses to How Many Fish? Beware of the Rule

  1. Gazali November 10, 2007 at 6:58 am #

    The rule is utterly nonsense.
    I have a 3 ft tank. My fishes are 7 Blood Parrots, 3 Red-tail Albino Tin foil barb, 2 Apollo Sharks, 5 Clown loaches, 1 Clown Knifefish and 1 Amored Birchir. As you can see plainly, the rule cannot apply here. Yet the fishes are thriving.
    There are 3 things that limit the fish population.
    1. Dissolved Oxygen Level
    2. Cardon Dioxide Level, and,
    3. Ammonia.
    I do not use an air pump. Instead I use a canister filter with the spray bar arrange to agitate the surface of the water. This is where the gas exchange takes place. Oxygen in and carbon dioxide out.
    How do I know if there is not enought dissolve oxygen in the first place? Its when the clown loaches (normally they are at the bottom) starts going up and also the rest of the fish.
    On the downside, I have to top up about 2 liters of water every 2 or 3 days.
    For ammonia it isn’t a real problem when the tank is fully cycled. My ammonia level is normally zero .
    My fishes are big and messy and I feed them pellets, flakes, feeder fishes like catfish, guppies, golfish and minnows and frogs too.
    I do weekly water changes of 36 litres every week.

  2. Eric November 10, 2007 at 2:38 pm #

    Thanks for your input Gazali! A great example of the “Rules” being tough, if not impossible to apply in some cases.

  3. Aaron February 10, 2008 at 11:44 pm #

    Hi my name is Aaron and I found that I was able to keep a large amount (by volume) of fish in a small tank- I have a Black skirt Tetra, two Zebra Danios, Two Oto Cats, and a Cory Cat. These are all in a 2.5 gallon tank and haven’t had any problems thus far- not unless you consider algae a spawning problem of too many fish. I do realize that it is a small tank for that many fish, but rest easy- I am in the process of putting them all in a ten gallon tank.

  4. Cook.MN April 14, 2008 at 7:42 pm #

    You realize Gazali that those Clown Loaches grow to over a foot and are best suited for tanks that have a 6 ft. foot print?

  5. Hevach April 19, 2008 at 12:36 am #

    I started my first tank without much consideration about rules and without knowing what I was getting into. I overstocked, as well as stocked too quickly, and to make matters worse, overfed.

    With some professional advice and extra work, I managed to get through it and keep it going. I’ve also been extra vigilant for signs of illness, since ick can spread fast enough in a less stocked tank.

    The fish are all small (Only a few over 4 inches and none over 5) tropical species, and are fed a diverse diet.

    Overfiltering (my filter is rated for up to 110 gallons, while the tank is only 30), using live plants, and more frequent water changes have all helped considerably. Reducing feeding was the biggest change.

    Like Gazali, I watch my bottom feeders as a warning bell: As long as they’re at the bottom and not stressing, I know things are under control.

    There’s added work and expense maintaining the tank, but disaster is avoidable. I’ve found the extra work quite rewarding, and there’s no denying that the result is very attractive.

    The one thing to make especially sure of, though, if you’re overstocking, don’t add overfeeding to that. Overfeeding can cause problems in any tank, but the more fish, the worse it can be.

  6. Eric April 23, 2008 at 6:08 am #

    Thanks for sharing Hevach! Your theme here is great advice. Much easier to time into learning and maintenance rather than fixing problems.

  7. Gazali May 20, 2008 at 9:50 pm #

    Yes Cook.MN, I do know how big some of the fishes will be. Don’t get me wrong. The 3ft tank is temporary. I’ll be switching to 5ft or 7ft tank soon.

    So far on growth rate, the clown Loaches is slow. The Clown knifefish grows faster and so does my ornate birchir (made a mistake, I thought initially I bought an armoured birchir).

    And I am now contemplating getting a lung fish. That fish can really be huge. Seen one about 3ft long.

    The Blood Parrots seemed to reached their max size and so does the Red-tail Albino Tin foil.

    Compatibility between the species is always an issue. I did have elephant nose fish before. Unfortunately it didn’t work out as the Blood Parrots was always chasing them. Had to house them in a separate tank.

    Agree with Hevach on overfiltering, regular water changes and less feeding.

    I use both a canister filter and an internal filter. And I do a weekly water change of about 36 litres. Mainly to vacuum out the waste accumulating on the substrate.

    And for the substrate, i use some crush coral mixed with common gravel. This is because there is a lot of waste (they are big fishes) and the water could turn acidic easily. So far the pH have stayed at 7.2.

    I feed the fishes once a day. Live fishes (about 30 or so) for the clown knifefish and birchir once a week.

    The thing is if you really want to have lots of fishes, you can’t be lazy. Your maintenance schedule must be followed.

    And as Hevach said, the extra work is rewarding.

  8. zore May 27, 2008 at 9:12 pm #

    ok, my question is not related to the subject.but i didn’t find any better place to post it. I’ve had my swordtail female and male for 6 months now and recently he started hitting her or attacking her while they were eating not letting her to eat. so we decided to add another female for him to decrease his aggressivity. but the first female died out of nowhere after 2 days of adding the new female. and he is still aggressive toward the new female and doesn’t let her eat. so what do you think we should do?

  9. Eric May 29, 2008 at 8:51 pm #

    Hi Zore,
    Swordtails are schooling fish. You really should have 3+, preferably 5. Many schooling fish have problems with aggressiveness without have the numbers. The numbers just let them establish a pecking order among themselves.

  10. alan June 4, 2008 at 8:45 pm #

    i am new to the indoor aquraium i am used to ponds but my wife just bought me a ten gallon tank for fathers day and two tigar oscars a plecustumus and a red bellied pacu i am concered that the room in the tank is in adacuate is this the case i know one day i will have to upgrade to a 100 to 200 allon tank but can they be comfortable for now thanks.

  11. Eric June 8, 2008 at 2:42 pm #

    Hi Alan,
    Only if they are an inch or so long. But they won’t that size for long at all. Your Red Bellied Pacu can get up to 2 or 3 ft long, Oscars can get up to 15 in and Plecos up to 2 ft or so. I would work very quickly to upgrade.

  12. Timothy June 9, 2008 at 4:00 am #

    In my 2 gallon tank, I’ve 2x Tiger Barb and 2x Rosy Barb….seems ok…….

  13. alan June 9, 2008 at 6:19 pm #

    thanks for your help i will try to get a larger tank in the future i changed out the gravel to some large beach pebbles and it seems to have helped this is a great sight. such quick response’s.

  14. Eric June 9, 2008 at 8:52 pm #

    You’re welcome Alan! Thanks for the kind words.

  15. Leslie June 18, 2008 at 2:06 pm #

    Hey, I have a 29 gallon tank. I have 3 ghost catfish, 2 mickey mouse platies, 2 cory catfish, 2 mollies, 3 fish I don’t know the names of, and…about 20 guppies!! As you probably know, guppies multiply very fast. I’m thinking of giving away my guppies and mollies and getting new fish. After I give them away I’ll have about 10 fish left. So, how many new fish do you think I could safely fit into my tank? Thanks!

  16. Eric June 23, 2008 at 6:02 am #

    Hi Leslie,
    The 10 you have left is probably ideal. 10 – 15 max 2″ adults with a 29 gallon is a good rule of thumb.

  17. Liz June 25, 2008 at 10:36 pm #

    Hi Eric,
    Apologies in advance because this question is sort of unrelated to the article but I didn’t know where to ask it.
    We have had our tank for about 7 months and it’s stable and I am quite good about weekly water changes and monitoring water parameters. We have lost a few fish here and there (about one every couple months), and almost always they die with no warning (no signs of illness or stress) and when we find the body, it’s totally mutilated. Today we found our algae eater in two pieces. It cannot have been more than 24 hours since he died, and probably a lot less than that. Usually I discover the body less than 6 or 8 hours after the fish died because I tend to peek into my tank a lot during the day.
    So my question is this: why am I finding perfectly healthy looking fish suddenly mangled like this? I have two rams and a tetra and a shrimp– I can’t imagine them attacking an algae eater and biting it in half. So what gives?

  18. Eric June 26, 2008 at 9:32 pm #

    Hi Liz,
    It sounds like your fish are probably being picked at after they die. This happens a lot. I don’t think those types of fish would be beating them to death. It sounds like you are really good about maintenance but could you be having some water troubles? I would start testing and looking for other signs of trouble in the tank.

  19. James July 1, 2008 at 6:24 pm #


    Ive a 76 litre tank and have had it set up happily for around9 months now. I do weekly waterchanges. I have had some trouble in the past couple of days. I am currently treating for whitespot as a recurrent problem in 2 of my black mollies. they are the only two that seem to be affected. i think i stopped the treatment too early last time which is why the problem reared its head again about 2 weeks after previous treatment ended. However i have just lost one black molly and am trying to save another black molly at the moment. The symptons are dropsy (swollen belly and raised scales) and a translucent faesces, long and stringy. Not moving so dont think it is a worm but i could be wrng. i saw this in both the dead fish and the one that is ill. I have treated this fish in a salt dip for 25 minutes and also put an air line in the dip to increase oxygen as the fish appeared to be gasping for air. I do not have another tank to use as an isolation tank unfortunately. When i tested my water i found the ammonia and nitrite to be slightly above zero which i know is not ideal. I have been doing a 30% water change before adding each treatment every 4 days. Could the treatment affect the water test results as i tested an hour after treating. Im using no 6 anti white spot treatment form lfs. Should i stop feeding and add stress zyme for a couple of days to try get these levels down?
    Also i had new molly fry that were still born this morning with ruptured stomachs, could this be a problem with the same cause? its the first time ive actually tried to catch the fry in a breeding box.
    I have slightly over the capacity of the tank if you use the rules to measure fish in inches to gallons, but not using the cm to litres, but some fish are only there temporarily before being moved to other peoples tanks. However i do not want to spread a problem to another tank. could someone please shed any light on this scenario. I have mollies, tiger danios, barbs and angel fish (the barbs get on fine with the angels as they have been around sail fin mollies that arent afraid to have a go back at the barbs if they try anything, but the angels are going anyway soon)

  20. Eric July 2, 2008 at 9:39 pm #

    Hi James,
    I would try getting a little more aggressive with the water changes frequency and % to quickly get them down. Like you mentioned, make sure you are adding your Stress Zyme each time as recommended. Also, are there any other places holding waste like an old filter pad? Gravel getting dug into well with siphoning?

  21. James July 3, 2008 at 2:39 pm #

    Hi eric.

    I’ve got the levels back to O ppm now which is good. stopped feeding for 2 days and added stress zyme each day with water changes. The issue i have is the illness that i am seeing in two of my mollies and the molly that died. They are lethargic and have a white translucent poo, almost tube like. It doesnt move so i dont think its a worm. Ive given the fish 2 salt dips now and the symptons of dropsy have dissapeared and they seem more lively, but im just aware that i dont know what im trying to cure atm and if theyres anything else i should be doing. Im halfway through a course of white spot treatment at the moment as well. However the problem of white spot has only ever been on two fish, none of the others are affected…?
    Any help on this matter would be great.

    (ps. do good thorough gravel cleans and make sure all filter pads are thoroughly cleaned in tank water on tank cleans to make sure theres no food matter or otherwise stuck in them)

    Thanks for the help. 🙂

  22. Eric July 3, 2008 at 9:21 pm #

    Hey James,
    I know how frustrating it can be when the signs just don’t pop out at you. Without seeing more, you might add some aquarium salt to the whole tank as long as you don’t have live plants or scaleless fish. 1 tsp per gallon will do the trick.

  23. Liz July 7, 2008 at 11:06 pm #

    Hi Eric,
    Thanks for your answer. I am discombobulated as to why my fish seem to be more fragile than I would like. Since my last post I lost a frog as well. I don’t understand since I do a 20% water change very week and I test for pH (7.8 very stable), Ammonia and nitrite (always zero) nitrate (usually about 7-8) and phosphate (.25) at most every water change. I feel like I am so good– why am I losing fish? I also have a lot more algae growth than I would like even though my nitrates are not that high and phosphates are very low. So I don’t get it. If there’s anything else you think I should test for, please enlighten me.

  24. Eric July 9, 2008 at 5:30 am #

    It does sound like you keep a general healthy tank. You might keep an eye on your heater/temp. High temperatures can promote algae growth and of course be rough on your fish.

  25. Jason July 9, 2008 at 8:27 pm #

    Hi all!

    I about to set up a 10 gallon tank and want some advice on kinds / numbers of fish. I’ve done some research and here’s what I’m thinking of doing:

    4 zebra danios
    3 platies
    1 red tale black shark

    And then after a couple of months I would probably add a bottom dweller.

    Maybe this is too much considering the various “rules” i’ve read about but there are all these comments about making sure certain fish, like danios, can school. What should I do?


  26. Eric July 10, 2008 at 6:50 am #

    That is a lot of fish for such a small tank. A red tailed black shark can get up to 6 in. My suggestion is to start adding fish slowly and maybe reconsider the numbers. After you get up to a few, give it some time while testing often. You’ll start to get a feel for it. You should be looking for the ammonia and nitrite spikes in particular. Take a look here for tips.
    The key to numbers is maintenance. The more you push the numbers the more frequently you need to do water changes and cleanings.

  27. Jason July 10, 2008 at 8:49 am #

    Thanks for the advice Eric. I will take all of what you said under consideration.

  28. Liz July 18, 2008 at 2:03 pm #

    Eric, I am at my wit’s end. I lost another fish today, this time my favorite blue ram. I don’t get it!!! I did a water test after I found him and I had exactly the same parameters as in my last post except this time phosphates were zero. My temp is a bit high (80 degrees) because the rams supposedly like it that way. But I have thin, dark algae growth all over all of my plants, which is very annoying. The only thing that I do that is not standard procedure is that I have my light cycle broken up to have four hours on in the morning, then four hours dark, then five and a half hours on again. This is so I can enjoy the tank while I’m home. Could this be the problem? I just can’t think of anything else.

  29. Eric July 20, 2008 at 12:14 pm #

    It wouldn’t be the light. Temp seems fine. With everything you have mentioned, here are some other suggestions. Look very close for signs of disease. Make sure you are using conditioner with water changes to rule out chemical issues. Look for bullying signs. Sometimes fish that are not normally aggressive can get that way. This would fit with your water conditions looking great. Make sure you have a new carbon filter.

  30. Nancy Smith January 1, 2011 at 6:39 pm #

    I think the rule 1 inch of fish per inch is a bunch of crud. You should just have each fish get 1 gallon of water it’s simple and it works so a 20 gallon fish tank could hold 15 medium fish because of decorations and gravel.