How Many Fish Can You Keep In A Fish Tank?


eclipsesystem.JPGMany times when starting a tank, the most popular and very important question is: How many fish can I keep in my aquarium? Of course there are many problems with over-stocking an aquarium such as insufficient oxygen, lack of space etc. Most of our exotic fish will take a lot of punishment before coming to the surface to whisper their complaints by breathing atmospheric air. They can be overcrowded without any such demonstration.

Many times the aquarium is in beautiful condition but the fish do not grow. This brings out the fact that the fish have neither enough “elbow room” nor oxygen and sometimes underfed. The rule of thumb I have heard many times is “1 inch of fish to 1 Gallon of water”. This, however, is only a rule of thumb, and many times it is hard to apply it to a specific fish.

For example, let’s compare a 3″ neon tetra, 3″ discus, 3″ tiger barb, 3″ betta, and 3″ cichlid. Each fish has different requirements. A 3″ discus would not be able to live in a tank suitable for a 3″ betta because of space. Even a 3″ neon and a 3″ betta have different requirements because the betta is a labyrinth fish. And if we take it even further, a 3″ barb and a 3″ neon need slightly different requirements as barbs are taller in body and neons slimmer. I could name many more examples but I think that you can see my point. Each fish has its own requirements.

The following then can be used to determine the number of fish in an aquarium. The first step is determining the surface area of the tank. Keep in mind when using the size of fish; it has to be the maximum growth the fish will reach.

For fish the size of grown Guppies, 3 square inches of air surface per fish. That is to say, an aquarium 9 x 20 inches, with an air surface of 180 square inches, can safely support 50 grown guppies, giving each over 3 inches. Other exotics the same size rate a bit more, as Guppies have great powers of standing overcrowding.

Labyrinth fish (Bettas, Paradise, Gouramies) need about half the amount of air surface calculated per fish of the same size.

Swordtails, large Platies, etc., need about 8 square inches per fish (4 x 2 inches, or equivalent).

Medium Barbs, 3.5 inches in size, and other fish of equal weight should have 20 square inches per fish (4 x 5 inches, or equivalent).

Large Barbs and Cichlids of 5-inch length require a minimum of 54 inches per fish (6 x 9 inches, or equivalent).

These are minimum requirements, not taking into account plants or aeration and at a temperature of about 74 degrees. For health, growth and first class conditions, the air-surface per fish should be doubled or even tripled. A large aquarium can support a little higher percentage of fish in proportion to its size than a small one.

By: Ron Reisdorf
**Data taken from ‘Exotic Aquarium Fishes’, 19th Edition Revised by Dr. William T. Innes

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{ 38 comments… read them below or add one }

Tamara October 6, 2007 at 2:41 pm

I was wondering if I have over crowded my 47 gallon, tall tank? I have 3 pretty big Angelfish, 1 Ram and 1 tiny shark with red fins. If so, which ones need to be moved to a larger tank? Thanks.

Johnny October 8, 2007 at 9:04 am

i’d suggest moving all of them to a bigger tank. ๐Ÿ™‚ you can’t ever have a big enough tank. but have you joined our forum? you can ask your question there and our helpful members can assist you.

Jamie February 6, 2008 at 9:53 am

I’m pretty sure my tank is overcrowded. My boyfriend continually keeps adding fish and I keep telling him that there are two many. Here is what we have we have 3 neon tetra, 4 African dwarf frogs, 2 Serpae tetra, 3 small dwarf gourami,2 bala sharks, 2 fancy guppies, 2 buenos aires tetra, 2 black skirted tetra, 1 dojo loach. Let me just say all of the fish are very small our largest fish are only an inch and a half to two inches in length. Which there are 6 of those that are in that size range. Should we purchase a larger tank? How large of a tank should we purchase?

Jamie February 6, 2008 at 10:12 am

I forgot about two little guys he just added hatchet fish.

Maryanne December 4, 2008 at 8:26 pm

Please do not perpetuate the myth that labyrinth fish such as bettas need less space than other fish. Bettas need at least 2 gallons of space per fish.

lexie January 25, 2009 at 5:28 am

I have a 5 footer so that saround 63 gallon, i think i over crowered it using the 1 fish to a gallon rule, but ive keepted it community, I have 20 neon’s, 5 glowlights, 7 serpaes, 15 Danios (mixed), 8 black widdows, 6 platties, 5 guppies, 2 Gouramis , 1 bronze catfish and 5 fantail goldfish, 3 other assored gold fish, do you think thats ok?

Wolf February 17, 2009 at 6:26 pm

One extremely predatory fish per tank is fun too.

izzasy November 23, 2009 at 8:43 am

While we do need to provide a required space for different species, the tank also needs to be habitat friendly for the fishes as well. The reason why we space is a requirement is the behavior of each fishes. It would be best to study the typical behavior of each fish (aggressive or passive). The aggressive types normally need more space to swim as compared to the passive. Description of behaviors of different types of fishes can be found here.

Amy March 9, 2010 at 7:03 pm

This was a really good article with lots of details. Hopefully it will help people make better decisions when stocking their tanks!

Anthony April 23, 2010 at 4:40 pm

While I think you bring up some important issues here, namely the inadequacy of the one inch per gallon rule, your article leaves allot to be answered. For example: what abou other medium sized or larger fish which don’t fit into your classifications? Also while you make a point of trying to prevent overcrowding, I assume for the purpose of maintaining water quality, you never adress overall swimming area. This is where I think Maryanne has a point. just because a betta won’t necessarily pollute a small bowl, and can survive in it for a reasonable amount of time, doesn’t make this a healthy option for the fish.

anonymous May 24, 2010 at 8:19 pm

You jerks stick a billion fish in 1 tank and think they’ll be happy? Guess what, they won’t! You are cruel, inhumane people I wish I could put 100 of you in a box and see how you like it jerks!

Farida June 6, 2010 at 11:57 pm

i want simple answer.
i want to keep betta fish.
Do let me know how many fishes i can keep in 1 gallon fish bowl and of what size of betta fish it should be.

Shauna September 13, 2010 at 11:32 pm

Hoping someone can confirm my calculations!? 60 gallon at 75-76 degrees / 305 Fluval exterior canister filtration / ‘tide’ maker to increase surface movement / 2 bubblers / caves / floating plants.

Surface area is 672 so the amount of 5″ fish comfortable wld be 12.4444444 ?

14 Sarassa comets / 1 pleco / 7 mbuna lake cichlids share tank. Currently have too many comets (14 in total) as brought in to over-winter inside. Smallest is 3″; largest is 7″. I took an ‘average’ size of 5″ for all fish to get the 12.44444 amount for the size of tank.

Yup, currently overcrowded but needing to know if the # I came up with is correct so I know how many to rehome ๐Ÿ™‚ Thanks everyone!!!

Jennifer January 22, 2011 at 10:02 pm

I wonder how big of a tank i need (gallons)?
I want to keep a few guppies and a few platys.
Will these fish get along?
I don’t plan on breeding any of them.

andrew January 30, 2011 at 8:22 pm

My friend hred me as her “fish expert” because shes saving up for a 55 gallon. she wants fish that get between a zebra pleco and a bala shark size. this is what ive come up with 1 bala shark, 1 zebra pleco(if we can find it) 1 angel fish, 1 leporinus fasciatus, 1 chinese algae eater, 1 african featherfin catfish and 1 kissing gourami. i no all these get along exept not sure about
1 the african featyherfin cat attacking the zebra pleco
2 the kissin gourami and the leporinus attacking the angel fish
3 i remember having 2 chinese algae eaters once living with an african featherfin and they dissapeared in about a week. I cleaned out the tank and couldnt find them anywhere.

sarah February 18, 2011 at 6:08 am

i have 12 fancy guppies 2 platys and 6 goldfish… what sized tank should i have? also 2 out of 6 of those goldfish are fantails… any suggestions?

Mike February 27, 2011 at 12:54 am

Yep. My 3 gall. currently holds 1 Otto 1 Neon Tetra 1 Apple Snail 2 Daino. Thats it.

Jess May 17, 2011 at 6:53 am

I have 3 goldfish in a 12″ fishtank, may I need to put some grass or green plants in it? I really like these fishes.

chris July 21, 2011 at 5:47 am

Hello every one, I have an 82 gallon tank with …
3 guppies, 3 platties, 2 swordtails, 2ghost catfish, 1 pictus catfish, 1 humbug catfish, 2 black widow tetras, a thew neon tetras, 4 corydoris, 2 weatherloach, a common pleco, a pleco (not sure what kind) ,2 mollies and a giant difformed dannio .. Is that okayyy???

Please tell me ….

chris July 21, 2011 at 5:53 am

And I have 2 baby fish… And its a 28 gallon*not a 82

Angela August 1, 2011 at 10:20 pm

A properly cared for betta (meaning water changes once or twice a week, and a steady temperature either by a heater or a reliable room temperature) can be happy in anything over one gallon. Anything smaller is NOT okay. If you want to go to extremes like betta fans seem to be doing these days yeah, 2.5+ gallons and a heater is nice, but he really doesn’t absolutely have to have that to be a happy fishy.

As for the rest of this article I do like some of the points you’ve made. Certain fish like guppies and neons are actually much happier in large crowds of their own kind. I always feel really terrible for neons when they’re kept solo or just as a pair. Those are very lonely little fish, I can guarantee. Still 3 inches of air is a bit stingy. I’d prefer a happy middle between that bare minimum and the old 1-inch-per-gallon rule.

jimi November 7, 2011 at 7:31 am

A few months ago, I brought a 125 gallon tank, and put in 14 cichlids of small sizes. Now they are getting big and I was wondering if my tank is going be two small for them. 4 Jack Dempsey/ 4 Servrums/ 1 Red devil/ 1 Oscar/ 1 Red terror

jimi November 7, 2011 at 7:37 am

A few months ago, I brought a 125 gallon tank, and put in 14 cichlids of small sizes. Now they are getting big and I was wondering if my tank is going be two small for them. 4 Jack Dempsey/ 4 Servrums/ 1 Red devil/ 1 Oscar/ 1 Red terror and 1 Firemouth cichlid. Plus an algae eater. So should I worry?

debbs November 27, 2011 at 1:06 pm

hello, i have a 170 litre tank for my tropical fish, and a 70 litre tank for my goldfish, 2 fantails and one yellow goldfish. how many can my tanks hold without overcrowding? many thanks in advance, debbs. UK.

Maddie December 30, 2011 at 7:58 pm

So I bought 2 neon fish and 1 shrimp will it do well??? I need to now soon!!!!! HELP!!

Bad boy January 28, 2012 at 12:42 pm

I have a 10 inch bowl how many fishes can i put,at present i have 20 fishes 8guppies 7mollies 2halfbeck 1sucker mouth and 2 small goldfish what will happen and plz help

mypippa1 March 5, 2012 at 12:46 pm

I have 2 fantail goldfish,both about 2 inches long and fully grown.I have them in a 25-30ish gallon tank.I want to get a couple more the same size so I was wondering if I would be ok to put maybe 2 or 3 more in there or if I will need a bigger tank.I want them to have a really good life.Thanks!

HootDay March 21, 2012 at 10:33 pm

Thanks for the great information. I am looking to get the fluval edge and i want a lot of fish in my tank but was worried about overcrowding. the tank is only 6 gallons and i didnt want to just get 3 fish due to the rule of 1 inch per gallon. I am glad i found your article to clarify the myth.

KETAN April 13, 2012 at 10:20 am


Laura November 4, 2012 at 7:37 am

We have an 50 gallon tank. In our tank we have one tiger fish, two male beta fish, one albino shark, two red tailed shark, two white tetra fish, two angle fish, one kissie fish, one bottom feeder which is getting bigger each month. Also in our tank have one big log with plastic plants, other items too. The sharks like to hide out in the log. We have had in there for about 5 months. Many people told us that the male beta’s could not live together and they get along like brothers. They we the first introduced the tank before the others. I am thinking that is why they get along…just my story tho.. not no expert..

Shane December 14, 2012 at 2:58 am

Just had a few questions as to how much space you all think I need for my livingstoni Cichlid? Currently I have a fluvul 40 gallon filter carbon and filter changes every two weeks along with a 20% water change and clean the gravel. I also have a 40 gallon air pump connected to a 10 inch air stone along the back of the aquarium underneath the gravel. I supplement with bio boost every month, my tank is a 30 gallon tall aquarium with led hood. Also my cichlid, that I have had for 6 months is 3 inches long and 1.5 tall. He seems very happy and energetic and playful in the set up I have for now. I try to mimic the natural environment of cichlids along with natural pebbles, plants, driftwood, rocks and plenty of hiding spots. When he is directly under the light of the hood you can see an array of a rainbow shimmer in all his fins contrasting his grayish white body covered in dark black abstract shaped spots. He is fed cichlids staple pellets and cichlids gold pellets, blood worms one or twice a week, and mysis shrimp. In your option do you think this set up will be suitable until I can upgrade the tank size before he gets over 5 inches. I’m planning on upgrading around 6 months or so. If you have any helpful information or care tips please let me know.
Thank you

Claire June 18, 2013 at 1:08 am

I can’t believe the shameless selfishness of fellow human beings! If you don’t provide enough living space for your fish they will get stressed & die young. An unhappy/wasted life for them, a waste of money for you. This is the shame of most gold fish owners! People buy or are given goldfish & put it in a tiny crowded bowl- then wonder why it dies after few months. Goldfish can live for 25 years, the oldest lived for 43 years! Think about this before buying a pet that may outlive your dog!

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Nikki June 16, 2014 at 2:27 pm

Ok I have a 16 oz tank I used to keep my old happy Betta in and I wanna know if I can take my 2 guppies out of my 10 gallon tank in it Because they are not having a happy time with 3 zebra fish and a rainbow shark and I heard guppies jump out but I just the fish expert my father doesn’t know anything he just the one who sets up the tank and stuff I need some advice it’s important + u haven’t had a comment in a year but please I need help NOW PLEASE

Nikki June 16, 2014 at 6:17 pm

Also my goldfishes have been been jumping out and one broke it’s back when it died I dissected it Nd disgusted it out anyway do stores carry some sort of transparent paper or something to let the light go into the tank but the fish do jump out? (I want to prevent this as quickly as possible)

senthil August 19, 2014 at 8:57 am

How many fish keep in 10inch bowl

BILL March 6, 2016 at 9:43 am



Johnny March 28, 2016 at 9:55 pm

hey there. if you have questions about this fish, join the forum at and the members there will be able to help you out!

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