I recently purchased two small goldfish for my son. We were instructed by the employee at Petsmart that the 2.5 gal (ca. 10 l) tank would be sufficient. I bought gravel, plants and water conditioner (stress coat).

Anyway, we've only been feeding the fish twice a day, and not excessive amounts of food. Their water looks cloudy, and they keep going to the surface for air. I just changed 50% of the water yesterday afternoon!

I'm frustrated that I spend $40 on these supplies, and from what I'm reading, I don't have a large enough tank. I don't have room for a 20-30 gal (75-115 l) tank. Should I return the fish or is there any way I could make this smaller tank work? Maybe for one fish?

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    U can't put any fish here, maybe snails or shrimp, but not a fish and ofc goldfish is the worst option.
    – Gawey
    Commented Apr 26, 2019 at 6:31
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    i want you to read(and understand this) thegoldfishtank.com/goldfish-care/tank/… the goldfish you get in petshops are fry and they will double in size in about two months so you will get big problems in a short time. Commented Aug 27, 2019 at 10:22

5 Answers 5


Return the goldfish and get a beta. That's realistically the only fish that can survive long term in a 2.5 gallon aquarium.

Your typical comet goldfish will need at least 30 gallons of water each to stay healthy long term. They're very messy fish and even in a small but suitable tank, owners often have major water quality issues.

Before you get another fish, read up on how to properly cycle a fish tank. If you take a fish tank, regardless of the size, and just put water and conditioner and fish in it, most fish will be dead in about a week due to ammonia poisoning. An aquarium must cycle before it is ready to house fish. Cycling is the process of building up beneficial bacteria. This bacteria is what actually filters the water in any fish tank, beit freshwater or saltwater. Cycling a tank typically takes around 4 weeks and is complete when ammonia and nitrite can no longer be detected in a tank, while nitrate is detected and stable or rising.

Petsmart is notorious for employing untrained people in their aquatics departments, and they routinely sell customers fish and aquarium setups that are impossible to maintain. In our aquarium club it's probably a weekly or bi-weekly event that someone shows up trying to keep their tank alive because petsmart or petco sold them a setup that was impossible to keep going, or even get established, and they've lost all their fish multiple times as a result.

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    Can you add references? 30 gallons per fish seems excessive and when I Google I get answers between 5-10 gallons each. Commented Feb 18, 2015 at 19:40
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    Here's one: tfhmagazine.com/details/articles/goldfish-myths-debunked.htm
    – Jestep
    Commented Feb 18, 2015 at 20:01
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    You should include the reference in the answer as comments get deleted. AND that reference does not support your quote of 30 gallons per fish. Commented Feb 18, 2015 at 20:19
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    The article states 55 gallons minimum, but preferably a pond. Since comets can get larger than 12 inches and several pounds, even a 30 is likely insufficient for an adult. However, 20 - 40 gallons is a generally suggested range that I've seen over the past 25 years of keeping fish. Personally I don't think they should be kept in anything but a pond. There's unlikely to be any scientifically relevant study on appropriate tank size so any reference is going to be subjective, which is why I didn't specifically include one. There is a number that specifically support the 20 to 40 range if needed.
    – Jestep
    Commented Feb 18, 2015 at 21:00
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    This tank is also too small for a betta, i would not recommend that out of respect for the betta. No aquarium fish should live in a 2.5 gals, this tank is for plants or snails. Even for shrimps it would be too hard to maintain the water quality and temperature. A betta might survive in this but it will not be comfortable and will die young. A tank of this size cannot maintain stable chemistry. I downvoted because this is perpetuating a harmful myth about how bettas can live in small bowls. Both betta and goldfish are though enough to survive bad water, both deserve better.
    – Manuki
    Commented Apr 23, 2019 at 15:58

Jestep has a good answer, but sometime it helps to be able to visualize things.

With the exception of a few show goldfish, which will only grow to about 10 inches in length, an adult goldfish is going to be about 14-18 inches long. You didn't say what species of goldfish you had, so I'm assuming it's the common comet goldfish that all pet stores sell. They will easily be at the upper end of the scale as long as you can keep them alive.

To put this into perspective, a standard 2.5 gallon tank is usually only 13 inches long1, although they can be shorter if you buy a special kit.

This means that for a tank to be big enough for a goldfish, you pretty much need to be able to wave a 2.5 gallon tank around the inside of it. Most tanks, aside from a 40g breeder tank, are only 13 inches deep, meaning the goldfish wouldn't be able to turn around without bending its spine too much. Goldfish are active swimmers so they need to be able to swim back and forth as they feel like it.

I would take the fish back to the pet store, they should have no problem with you returning it, although they might only give you a refund if it's been within a certain time frame. You could probably get a betta for the tank, although in my opinion a 2.5 gallon tank is still a bit small for them. I would personally turn it into a nano tank with some plants/moss and some shrimp.

That's personal preference though. What you'll really want to do is look around at your local pet store, and see what fish they have available. Then be sure to look up how large the fish grows to as an adult, and if it's a schooling fish that will require tankmates. Don't be afraid to visit just to write down names to look up at home.

1For reference, here is a site with a list of tank dimensions based on their size in gallons/liters: http://www.anapsid.org/resources/tanksize.html


There are a few factors in determining tank size. One of the biggest factors is surface area. The way it works is that fish pull oxygen from the water. The less surface area there is, the longer it takes for the oxygen to exchange from the surrounding air into the water. If you have too many fish breathing, then they use up all the oxygen. As an experiment, put your self completely under a heavy blanket. After a while, you'll notice that it becomes harder to breath. This is because you're burning up the oxygen faster than it can come through the cloth. That's what will happen to your fish. They'll slowly asphyxiate if there isn't a good surface area to fish ratio. You can find many calculators to help you with that. Here is one: Fish Calculator

That's just the oxygen. The next is water quality. Both can be improved by circulation through a filter. Fish are messy animals. They waste large portions of their food, they pee and poo everywhere, and generally make a mess. Just like a person living in their own filth, it will eventually effect their health. They can't help but come into contact with their own waste, so they aren't going to sicken and die like a person might when exposed to cess. However, it's not as hard as it sounds. In the wild, there are all kinds of things helping to process that waste out of the water, from sun, to other animals, to plants (who not only clean waste, but produce oxygen). To fix this in a closed system, you have filter systems. These suck up the solid waste to a degree, filter the water to remove ammonia and other harmful chemicals, and return the water, which agitates it and helps put oxygen back into it. You can add bubblers and real plants to help deal with these issues as well.

The fish will also continue to grow throughout it's life, since fish have something called indeterminate growth. You can look up the adult size of your fish and determine if there is enough room in your tank. I found two good articles here: http://injaf.org/articles-guides/do-fish-grow-to-the-size-of-their-tank/ and http://injaf.org/articles-guides/understanding-fish-stocking-guides/

So to sum it up, there a various factors that determine the fish that can go in a tank. Factors to keep in mind are adult size, oxygen levels, waste removal, and aggression, though that probably doesn't apply to you. I think that at minimum you'll want a tank with some type of filtration system and not one that's stagnant. You could also get away with a smaller tank if you traded in your fish when it started getting too big for the tank. I can tell you that I've done that many times. A pet store will often give you smaller versions since they can make more money off of selling your larger fish. If not, you have a way to get rid of your larger fish and keep the smaller.

Something I'd recommend is getting a 5-10 gal tank (many retail stores and pet stores sell a 5gal hex with a built in filter, and buy some naturally smaller fish. I like the small colorful ones anyway. Guppys are great. They're super colorful and are small. They also breed easily, so you can have babies. That's a different can of worms, though. You can also get neon tetras, platys, swordtails, mollies, etc... You can also get a cool pet like a crayfish or fresh water crab. They don't need much space, just a place to hide (they'll create one if you put an object like a hide in their tank), flowing water ( a filter return is fine), and food to eat. Mine ate fish food, fish waste, and the random muscle I would catch at the river. I'd catch them at the creek and keep them for a couple of years. They molt and get bigger every couple of months. Another cool feature is that they turn colors to match their surroundings. Not like a chameleon, but I caught one that was a 1/4" long and it was jet black. I had white gravel in the tank. He got lighter at every molt and by the time he was 2-3" long, he was blue-white. He started killing my fish and had to go, but he was cool and I caught a smaller one to replace him. They clean fish waste and can be kept with fish if there is plenty of room. They are predators and will hunt them eventually. Good luck.

  • "Fish calculator" is not a good tool, some kind of fish need "special" space depends in their requirements. For example: Betta or Ramirezi are extremely territorial with other fishes.
    – Gawey
    Commented Apr 26, 2019 at 6:55

I do not have experience with golden fish, so I cannot help there. However, you also state:

Their water looks cloudy, and they keep going to the surface for air.

in the following context:

I recently purchased two small goldfish for my son. We were instructed by the employee at Petsmart that the 2.5 gal (ca. 10 l) tank would be sufficient.

This means that the aquarium is not properly set, and you do not have much experience.

I answered shortly what does it mean to have an aquarium here.

The most important device that you need is the mechanical filter.

The most important activity at the beginning: cycle the aquarium.

If you do the things properly, in the end you will only need to add water when it evaporates, and to clear the excess dirt in the mechanical filter.

What the other friends here told you is true: if the aquarium is too small / overpopulated, you will never achieve a good balance.


I'm gonna get yelled at by online people but I got two small goldfish (an inch or less each at the start) and kept them for 12 years. First couple of years they were in a 3 gallon tank, then moved to a 5 as they grew, finally a 25 as they were both about 4-5 inches at the end. They were active and happy and never had any health issues at the end. I even consulted vets. Just saying you don't HAVE to have 10 gallons per inch of goldfish, that's what I read a lot on unaccredited websites.

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    under good conditions goldfish will grow to about 20cm in one year so this is a bad example. Commented Nov 3, 2019 at 6:02

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