How Many Fish Should You Put in Your Tank?

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It’s no denying that fish-filled tanks are more impressive than those with one or two specimens floating around in a depressive habitat. It’s also true that many novice fish keepers tend to value esthetics more than practicality and crowd more fish into their tanks than they’re supposed to.

This leads to overcrowding which is a common occurrence in the aquarium world. Overcrowding can lead to a variety of problems. Some of them include:

  • Spikes in the ammonia levels – The more fish you have, the more poop they will produce. The fish’s waste will gather on plants and bury in the substrate if the substrate’s granules are large enough. The residues will release ammonia which will also spike the levels of nitrites, hurting your fish as a result. Ammonia is deadly to fish if left unchecked.
  • Fish stress – Fish can experience stress under certain conditions. These include unfit tank conditions (ammonia buildup), bullying, disease, and, of course, overcrowding. Overcrowding especially will create a lot of issues since most fish don’t feel comfortable in tight and crowded spaces.

These 2 problems alone show why overcrowding is such a dangerous situation to have. But what can you do to avoid overcrowding? This is what we’re here to discuss.

Factors to Consider When Getting Pet Fish

Setting up an aquarium takes planning and getting the right fish for your tank. Several criteria should come into play when deciding how to accommodate your fish to their new environment. These include:

– Tank Size

The tank size is the first metric you should be interested in. The problem is that most novice fish lovers tend to get it wrong, and they will always rather sacrifice the fish’s comfort rather than the tank’s size. In other words, they want a small tank investment while expecting to get as many fish as possible.

This will naturally lead to overcrowding and trigger the problems I’ve mentioned earlier. Figuring out which tank size to go for goes hand in hand with choosing the right fish species for it. That’s because every fish species has different tank requirements. To give you several examples:

Clownfish requires a 10-gallon tank per individual and a 60-gallon tank per schooling

The Oscar requires a 50-60-gallon tank per individual and 150 gallons for 2-3 specimens

Guppies require 2 gallons of water per fish and a 10-gallon tank for a schooling of 5 fish

Royal Gramma needs at least 30 gallons of water and 100 for a schooling

Then you have 2 other aspects to consider:

  1. The fish’s growth rate – Some fish will start off small and grow to impressive proportions fast. Take the Oscar, for instance. This giant will grow up to 1 inch per month during its first year of life and will need a considerable tank upgrade along the way. This shows that it’s always a bad idea to purchase the tank for small, undeveloped fish, disregarding their growth rate and adult size.
  2. The fish’s reproductive ratesSome fish are extremely prolific breeders with the potential of creating dozens or hundreds of offspring every month. Guppies fall into this category. One female guppy can give birth to 2 to 200 fry every month, with or without a male present. Guppy females can store the male’s sperm and use it to self-impregnate for up to 9 months in a row. If you’re not taking the fish’s reproductive behavior into account, your tank will become overcrowded fast.

These issues clearly show that choosing the tank size carefully is essential for preventing overcrowding.

– Fish Species

Choosing your fish species carefully also makes a significant difference in keeping your fish healthy in the long run. Not all fish function the same. Some have strong schooling behavior like Cardinal Tetras, others are somewhere in the middle, like guppies, and others are loners, like the Oscar.

Each fish species will require specific tank conditions to accommodate their biological predispositions. Oscars, for instance, require 50 gallons of water per fish not because of their size necessarily, but rather their behavior. The Oscar will reach around 12-13 inches, and they are inquisitive and intelligent fish who need constant mental stimulation.

A larger tank will provide them with the opportunity to explore their environment and keep themselves occupied.

Other fish don’t need too much space but require plant-rich environments to remain comfortable and healthy in the long run. You should inform yourself on your preferred species before buying it to make sure you can meet its environmental expectations.

– Filtration

A filtration system is absolutely necessary in any tank. The filter will:

  • Cleanse the water of debris, dead matter, flying particles, and other organic compounds
  • Improve oxygenation
  • Control the levels of ammonia and nitrites, keeping the environment clean and healthy
  • Create water current which helps some fish species breathe better
  • Minimizes the need for frequent tank cleaning, which will stress your fish in the process
  • Removes food residues floating in the water that your fish won’t consume anymore, reducing the impact of overfeeding
  • Makes the water clearer and appear cleaner and more transparent overall

You can also go without a filter, but you need to fill its role. No-filter tanks require more regular cleaning, which will stress your fish. And it’s never a good idea to stress your fish constantly since it weakens their immune system and leaves them open to diseases and parasites. Get a filter!

– Tank Maintenance

Filter or no filter, you still need to perform regular tank maintenance. There are typically 3 cleaning phases to consider:

  1. Occasional cleaning and vacuuming – Some people only clean their tank every 6-8 weeks and avoid touching it in the meantime. Others also perform some minimal cleaning along the way. Be like the latter. This means cleaning the fish waste and food residues occasionally with minimal disturbance to your fish. We’re talking about some superficial cleaning designed to remove the bulk of the residues without going in-depth.
  2. Water changes – You need to perform weekly water changes to keep the aquatic environment stable and clean. A 10-15% water change is necessary to freshen up the environment, boost the water’s oxygen levels, eliminate ammonia, and restabilize the tank’s biofilm. Do not go overboard with water changes since there are no shortcuts to the procedure. You can’t change 30% of the water this week to skip the following week. Changing too much water at once can destroy the cultures of beneficial bacteria and will dilute the minerals in the water that all fish need to survive.
  3. Generalized cleaning – The last cleaning phase should be more thorough and will take place every 6 to 8 weeks. The procedure relies on cleaning the tank’s walls of algae, removing visible fish waste and food residues, vacuuming the substrate, and removing any dead fish and dead plants. This is a necessary phase in the cleaning process as it will prevent the dangerous buildup of ammonia due to a dirty and murky environment.

– Live Aquatic Plants

Aside from their esthetic appeal, live plants provide your fish population with a variety of benefits. These include:

  • Inhibiting algae growth since they compete for the same space and resources
  • Using photosynthesis to produce oxygen during the day, keeping the water well oxygenated for the fish
  • Providing fish with safety and comfort as they use plants to hide, rest, play and even mate
  • Providing fish fry with shelter to prevent adult fish from hunting and eating them

Live plants are essential to any aquatic environment, especially an enclosed one like an aquarium.

How Many Fish Should a Beginner Get?

As a beginner, you should focus on 2 aspects when getting your fish:

  1. Getting a less-demanding species – Some fish species have special water requirements and dietary and care needs, requiring you to invest more time and effort in their care. Other species tend to be more sensitive to environmental fluctuations, leaving them prone to various conditions. If you’re a novice in the business, avoid overly demanding fish species to prevent running into complications along the way.
  2. Consider the number of fish – The more fish you have, the more maintenance you will need to perform. As a beginner, you should get a small number of fish, allowing you to care for them easier. Also, don’t throw yourself at a large community tank. Keep it simple at first and get a 10 or 20-gallon tank only with several fish and upgrade it later on, once you’ve become more experienced.

I see a lot of novice fish keepers making large initial investments and finding themselves in deep waters fast. This will lead them to become discouraged and lose their fish in the process. Act smart, start small, have patience, and you will soon become a proficient fish keeper.

Is it Better to Have One Fish or More?

The answer depends on the species you’ve set your eyes on. Some fish like to live alone like the Oscar. Other fish only tolerate members of the opposite sex, like Bettas. Betta males, for instance, will become aggressive, territorial, and combative in the presence of other Bettas.

Then you have schooling fish that only feel comfortable in groups like many species of tetras, rasboras, guppies, etc. The latter will occasionally display schooling behavior when threatened or when sensing predators nearby.

Generally speaking, it’s easier to care for one fish than more, but this is not always the case. The Oscar, for instance, is quite demanding since it requires pristine environmental conditions, as well as regular entertainment. These are energetic and inquisitive fish that require mental stimulation to remain happy and healthy over the years.

On the other hand, guppies are less demanding and easier to care for, despite having multiple fish in the tank. From a novice’s perspective, I would focus on getting easy-to-care fish in numbers that I can manage comfortably.

I you love guppies, get a group of 6 or 7, for instance, 2 males and 4-5 females. This approach should ensure stable group dynamics and will allow you to care for the fish with minimal effort.

Is it Bad to Have Too Many Fish?

Yes, it is. Keeping too many fish will always come with various problems, which we’ve already discussed in the beginning. But what if you have enough room for them? That will only remain a problem if you are a beginner in the industry. The more fish you have, the more effort you will have to put into their care.

Other than that, having too many fish can become quite productive. Selective breeding is always an option in these cases, which can work great if you own sought-after fish species.

Just know that you will have to keep multiple tanks to accommodate your fish and separate them by gender, strain, and generation. I have already written a comprehensive fish guide about selective breeding, teaching you everything you need to know about the issue.

How Many Fish to Put in a 10-Gallon Tank?

Many fish lovers will get 10-gallon tanks at first and build from there. A 10-gallon tank, however, will only allow you to get certain fish species that don’t require too much space. As a general rule, allocate around 1 to 2 gallons of water per fish, depending on big the fish is.

Here are some fish that work great with a 10-gallon tank:

  • GuppiesGuppies are ideal for 10-gallon tanks since they don’t need too much space compared to other species. You can easily house 4-6 guppies in a 10-gallon tank, along with plants and various decorations. Guppies are also easy to maintain and are hardy and adaptable, making them ideal for beginner and experienced fish lovers.
  • Corydoras – This is a bottom-feeding species that will be at home in a 10-gallon tank. The Corydoras won’t grow more than your typical guppy, show schooling behavior, and prefer swimming near the substrate, feeding on food residues from mid and top-dwelling fish.
  • Neon Tetra – Tetras are small, energetic, schooling fish that will thrive in a 10-gallon tank. A 10-gallon Tetra-only tank can hold as many as 10 fish, so long as you provide pristine environmental conditions.

How Many Fish to Put in a 55-Gallon Tank?

It depends on the fish species. If you plan on setting a guppy-only aquarium, you can easily place 25-30 guppies in a 55-gallon tank. If you’re aiming for a community tank, you can add several fish from each species, depending on their water volume requirements.

Or you can keep fewer but larger fish. One Oscar is enough for a 55-gallon tank, for instance. If you want 2 Oscars, you should double the tank in size.


How many fish you get for your tank depends on several criteria. These include:

My advice would be not to overcrowd your fish. There’s nothing good to expect from it except poorer water quality and your fish experiencing stress, bullying, and sickness.

Author Image Fabian
I’m Fabian, aquarium fish breeder and founder of this website. I’ve been keeping fish, since I was a kid. On this blog, I share a lot of information about the aquarium hobby and various fish species that I like. Please leave a comment if you have any question.
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