125 Gallon Aquarium Guide: Stocking Ideas, Size and More

A 125 gallon aquarium is perfect for beginners and experienced aquarists alike. Although they are commonly considered to be harder to maintain, this is not necessarily true.

When it comes to stocking a fish tank of this size, there are many species to choose from and throughout this article we will provide you with various popular options to help you get started.

Depending on the setup you choose your aquarium is sure to be lit up by lots of different colors and species.

In this article we will discuss all the essential things you need to know before buying a 125 gallon fish tank including stocking options, the expected cost, the equipment needed to maintain them and a lot more…

125 Gallon Aquarium

125 Gallon Aquarium Overview

The larger the aquarium, the more options you have when deciding what to do with them.

So a 125 gallon is the perfect example with it being one of the biggest home aquariums available on the market.

125 gallons of water will provide plenty of space for keeping a variety of fish. It also means that you will be able to keep larger or more territorial species which are not suitable for smaller tanks. Some of the most beautiful fish to include in a tank of this size are Bettas, Tetras or Tangs.

Not many people realize that it is easier to maintain larger aquariums’ water chemistry. The more water there is the more stable the water parameters become.

This means that no matter your experience level these tanks are great for beginner and expert hobbyists alike.

Most 125 gallon aquariums’ dimensions are 72W″ x 18D″ x 22H″, however the exact dimensions can differ slightly depending on the shape or brand of the tank.

When empty the tank will weigh around 206 pounds, but once full it can weigh anywhere up to 1200 pounds depending on your setup.

How Much Does A 125 Gallon Fish Tank Cost?

125 Gallon Fish Tank

A new 125 gallon aquarium will set you back somewhere in the region of $800-$3500.

The price will vary depending on several factors including whether or not you are purchasing the aquarium as a kit, which will come with items such as a hood and a stand.

Another factor which can influence the price is the material the aquarium is made from. Aquariums of this size come in two different materials: glass or acrylic.

Glass aquariums are cheaper and acrylic tend to be more expensive.

Acrylic is very easily scratched whereas glass is more durable but also much heavier.

Another factor which will impact the price is the shape of the fish tank. There are three main options to choose from: classic rectangular shaped tanks (cheapest), semi-cylindrical ones and even pentagonal aquariums (most expensive).

125 Gallon Fish Tank Stocking Ideas

Discus Species Only Tank

There are a lot of different species to choose from when you have a 125 gallon aquarium.

Because you have more space than other aquariums, you can add more fish.

A 125 gallon aquarium is perfect for freshwater tropical or cold water fish as well as many marine species.

Below we share 5 different setup ideas perfect for a tank of this size.


Angelfish Tropical Tank

Tropical species are the most diverse group which means picking the correct fish for your aquarium can be quite a challenge.

Here are a few examples of fish which are ideal for a 125 gallon tank:

Clown Loach: This is a colorful and social species of Loach that prefers to live in large shoals. They can reach a size of around 12 inches and prefer water temperatures of 78-87°F.

Discus: Discus are vibrantly colored fish which can grow to around 6.5 inches long. They can be kept with lots of other fish, however smaller species of Tetra are the most common tank mates.

Freshwater Angelfish: A stunning member of the Cichlid family which can be kept with Plecos as well as other smaller species like Mollies and Platy fish.

Rainbowfish: They are perfectly named as they come in a variety of different colors. They can be kept with a variety of peaceful species including both large or small Tetras and Corydoras Catfish in water with a temperature of 74-78°F.

Neon Tetra: A spectacularly colorful species which prefers to be kept in large shoals. A tank of this size could easily keep a shoal of up to 100.

Bettas: Both males and females can be aggressive and territorial. A tank of this size will provide enough space to keep either a sorority of females or a single male, with fast, peaceful fish like Ember Tetras.

Guppies: This is a live bearing species which are certain to breed like crazy. They come in lots of different colors due to selective breeding. Guppies are a very peaceful fish which should be kept with other fish of a similar nature.

Bristlenose Pleco: A nocturnal bottom feeding fish which would make a great addition to any community tank. They grow to a size of up to 5 inches and require a water temperature of 60-80°F.

Read 25 Most Popular Freshwater Fish For Beginners for more stocking ideas.


Clownfish Marine Tank

A fish tank of 125 gallons also provides you with the opportunity to set up a marine water ecosystem where many different species can be kept.

There are a variety of species to choose from including:

Ocellaris Clownfish: The Common Clownfish is a charismatic little fish which grows to around 4 inches. They can be kept with fish like Damselfish, Tangs, Angelfish and many others. Just keep the water temperature between 75-82°F.

Yellow Tang: This is a popular marine species with a bright yellow coloration. They prefer to be kept in small groups and get along well with other peaceful species like Fox-Face and Clownfish.

Royal Gramma: They are a small colorful species which grow to around 3 inches. Their vibrant purple and yellow color is perfect to brighten up the aquariums of experts and beginners alike.

Blue Tang: These beautiful fish can reach a size of up to 9 inches in length. They can live in a tropical marine aquarium with a temperature range of 73-82°F and can be kept with other peaceful species.

If you need more suggestions then read Top 21 Most Popular Saltwater Aquarium Fish.


There are not as many cold water fish to choose from, but some good picks include:

White Cloud Mountain Minnows: This is a shoaling species that looks similar to Neon Tetras. They are also available in golden and long-fin varieties and prefer a water temperature of 60°F.

Hillstream Loaches: An algae bottom feeding species that prefers faster water flow and a temperature of 60-75°F.

Paradise Fish: This is a vibrantly colored little fish. Males can be territorial so it is recommended that a single male is kept with multiple females. Water temperature should be stable in the region of 50-71.5°F.

Also don’t forget about Goldfish!

Community Tank

An aquarium with lots of different species living in it is referred to as a community tank.

With this type of tank you can keep your favorite species in one tank, providing they are compatible with each other and share the same water parameters.

The key to a good community tank is to have different species which occupy different levels and parts of the tank. To get this balance right you should choose fish which inhabit different regions of the tank.

One example could be to have Common Hatchet fish who stay at the surface layer of the water. Then take fish like Congo Tetra who will occupy the mid-regions of the tank. Mid-level fish such as a shoal of Neon Tetras will also work well.

Bottom dwelling fish like Plecos or Corydoras will act as a clean-up crew and eat any uneaten food and algae.

Species Only Tank

One kind of aquarium you might choose is a species only aquarium.

This is where you will keep a single species within one tank.

A species only setup is often used for species like Piranhas or other predatory fish, as well as territorial species like Oscars. Live bearing fish species like Mollies, Guppies and Platys are also kept in species only aquariums in order to keep selectively bred strains pure.

How To Set Up A 125 Gallon Tank

Goldfish Coldwater Tank

The first thing you should do before adding anything to your aquarium is to give it a thorough cleaning.

Once the aquarium is clean you are going to need a couple of extra hands to help you place the tank into its final resting place. This should ideally be on a stand away from any windows which receive natural sunlight.

When in position you can begin to add things to the tank.

The first thing on your list should be the substrate. Make sure it is cleaned correctly to avoid cloudy water. Add a 2 inch layer of substrate and spread it out evenly throughout the aquarium.

Next you need to add any ornaments or hardscaping materials like rocks and bogwood to the aquarium before adding the water. The reason for this is that if you fill the tank up prior to adding decorations then they will displace the water and can cause the tank to overflow.

After adding both the decorations and water to the fish tank, the water now needs to be treated with a dechlorinating solution to remove chlorine and chloramine.

With the substrate, decoration and water now added to the fish tank, the next things you are going to want to add are a filter and a heater.

Cycling The Aquarium

The second stage of preparing the aquarium for your fish is to cycle your tank.

This step is the most important one in the setup process because as fish excrete waste they produce ammonia which beneficial bacteria then turn into nitrites. A second type of bacteria then converts these nitrites into nitrates.

Ammonia and nitrites are toxic to fish and other organisms whereas nitrates are non-toxic if kept at lower quantities. A great way to keep nitrates low is to have a planted aquarium.

Plants will absorb nitrates as it is an essential nutrient required for growth. High quantities of nitrates in the aquarium with minimum live plants will result in the growth of unwanted algae. When plants or algae absorb nitrates, they complete the nitrogen cycle.

Read How To Cycle A Fish Tank: The Complete Beginner’s Guide.

It should take around 6 weeks to fully cycle – a similar time to the 75 gallon aquarium.

Adding Fish

By this point you should have chosen the different fish species and/or other inhabitants.

You now need to make sure that your nitrogen cycle is fully completed and that both the water temperature and pH are suitable for the fish you intend to keep.

Once these checks have been completed you can start to add your fish to your 125 gallon aquarium.

The first thing you should do is place the transport bags into your aquarium to begin acclimating your fish to the temperature of the aquarium. This helps to avoid shocking your fish and they should be left there for anywhere between 30 minutes to 1 hour.

When the water temperature in the bags is the same as the tank, you should then open the bags and start to fill your aquarium water into the bag. A ratio of 1 cup of aquarium water every5 minutes works well.

It will probably take around 30 minutes until the transport bags are full of aquarium water.

After this you should place a clean net over a bucket and pour the contents of the bag into the net. The water will pass through the net and into the bucket leaving only the fish inside. Now you should place the fish into your aquarium.

Invertebrates like shrimp will need to be drip acclimated inside of a bucket using a small siphon for a period of up to two hours as they tend to be more sensitive to temperature and pH shocks.

How To Maintain A 125 Gallon Fish Tank

125 Gallon Tank Maintenance

A 125 gallon tank will require the same maintenance processes as other smaller aquariums although they will take longer due to the larger size of the tank.

A weekly 20-30% water change should be performed. At this time you can also clean the gravel and remove any fish waste.

Cleaning the filter is another maintenance chore required for any aquarium. To do this, you simply need to rinse out any filter media or sponges into a bucket of aquarium water to remove any waste. This should be done every 3-4 weeks.

Another common maintenance job you will need to carry out is algae cleaning. This should only be done once every few weeks, but if the tank receives natural sunlight then it may need to be carried out more frequently. Cleaning unwanted algae off the tank can be done using either a magnetic scrubber or a simple clean dish sponge.

You can sometimes run into the problem of green water. This is due to an imbalance of nitrates and/or lighting which causes algal blooms within the water. There are a couple of ways to prevent this. First, you can purchase algae destroying chemicals which work great but may harm live plants. Or, you can keep lights in the tank switched off for a few days and keep the tank covered with towels to prevent light getting in.

Should You Get A 125 Gallon Aquarium (Summary)

A 125 gallon aquarium gives you lots of space to keep lots of freshwater, brackish or marine species.

They require the exact same maintenance as any other sized tanks although some tasks may take a little longer. Also due to the size of these aquariums and as with all fish tanks, each gallon comes with an added cost. Generally speaking larger fish tanks are more expensive.

The good news is that with larger aquariums it is easier to maintain stable water parameters.

They provide plenty of room for various possible setups ideas and different fish.

How would you stock a 125 gallon aquarium?

Let us know in the comments section below…