A 40-gallon tank is a good choice for beginners and experienced keepers alike. It is big enough to hold a stable community, a planted layout or a single large display fish, and still fits in most living rooms.
It holds enough water that temperature and water chemistry change slowly, so it is more forgiving than a 10 or 20-gallon tank. It is also large enough for an aquascape with real depth.
This guide covers the types of 40-gallon tanks, equipment, stocking ideas, aquascape ideas, and how to set one up.
See also: Female Betta Fish
All About The 40 Gallon Aquarium
A 40-gallon aquarium is one of the most flexible sizes in the hobby. You can keep a wide range of small to medium-sized fish, aquarium plants and decorations, and it works as a first tank or a long-term display.
Two shapes are most common:
- 40-gallon breeder: about 36 x 18 x 16 inches (91 x 46 x 41 cm). It has a wide footprint and is not tall, so it is easy to reach into and good for aquascapes and territorial fish. Most hobbyists consider it the best all-round shape for this volume.
- 40-gallon long or tall: a longer, narrower tank or a taller one. Check the dimensions before buying a stand or lid, since 40-gallon tanks are not all the same size.
Other shapes exist, such as hexagon and corner tanks. They can look striking, but they are harder to clean and often limit your layout. A classic rectangular tank is easier to clean, easier to light, and the usual choice.
Water alone in a 40-gallon tank weighs about 334 pounds (8.34 lb per gallon), and gravel, rock and the tank itself add more. Put it on a purpose-built stand on a level floor, ideally against a load-bearing wall.
A 40-gallon tank suits both freshwater and saltwater, though this guide focuses on freshwater.
40 Gallon Fish Tank Equipment
Aquarium water chemistry changes with temperature, flow and waste, so the right equipment keeps conditions steady.
Filter
The filter keeps the water clean and moving. A filter also adds flow, which many fish need.
Choose a filter rated for at least the tank volume. A common rule is to turn over the tank volume about 4 times per hour, which means roughly 160 gallons per hour for a 40-gallon tank, and many keepers go higher with messy or large fish. Match the flow to the fish, since bettas and some gouramis prefer gentle flow. Options include sponge filters, hang-on-back filters and canister filters, and you can read about filters that suit your budget.
Heater
Most freshwater community fish need stable, warm water. A big temperature swing can cause health problems.
This simple device keeps the water at a set temperature. For a 40-gallon tank, a heater in the 100 to 200-watt range is common, depending on how cold the room gets. Many keepers use two smaller heaters instead of one, so one failure does not crash the tank. Use a separate thermometer to check the heater.
Read this Article: Pygmy Cory: Size, Tank Mates, Care, and More…
Lights
Light drives growth in plants and gives fish a day and night cycle. Too little light starves live plants. Too much light, or too many hours of it, lets algae spread quickly.
A timer set for about 6 to 8 hours a day is a good start. Increase the time only if your plants need it.
Other Equipment
An aquarium stand is a must for a tank this size, because it supports the weight and gives you space for equipment. Fitting your tank with custom aquarium hoods or solid covers helps reduce water evaporation, maintain temperature stability, and prevent active fish from jumping out.
You will also want:
- a liquid test kit (ammonia, nitrite, nitrate, pH)
- a gravel vacuum for water changes
- a dechlorinator
- a net, a bucket used only for the tank, and an algae scraper
Automatic feeders and magnetic cleaners are useful extras.
What Fish Can You Keep In A 40 Gallon Tank (Stocking Ideas)
A 40-gallon tank has a long list of species it can host, and some of the most popular fish that you can keep include:
| Fish | Description |
| Guppies | A favorite of many aquarists and a great beginner fish. In a species-only 40-gallon tank you can keep about 20 of them, with one male to every two or three females. These colorful little fish are really active, but also very peaceful and need to be kept in groups of at least 6. |
| Cory Catfish | These lovely small fish are entertaining bottom dwellers that sift the substrate for leftover food. They do not eat algae, so feed them sinking food. You can fit as many as 10 of them in a 40 gallon tank, in a group of at least 6. |
| Banggai Cardinalfish | An incredibly beautiful and unique addition to a saltwater aquarium (they are not a freshwater fish), these fish have a very peaceful temperament that is so desirable in many community species. A 40-gallon tank suits a single fish or a pair; a group of five or more needs a larger saltwater tank, roughly 55 to 75 gallons. |
| Betta | These extravagant fish are beautiful, active and hardy. They are quite aggressive and males should be kept on their own. |
| Zebrafish | With their vivid appearance and almost endless diversity they should not be overlooked. You can keep around 20 in a 40 gallon tank. |
| Convict Cichlid | This large fish is very active but loves to spend most of their time hiding in crevices or under rocks. Convicts are aggressive and territorial, so keep a single pair in this tank. |
| Mollies | These beautiful fish never stop to impress despite their relatively modest size. The Molly is an interesting fish whose energetic behavior in the tank will keep you entertained for a long time. You can keep 8-10 of them in a tank of this volume. |
| Goldfish | Perhaps the most iconic fish in the world, this shining beauty is a dream for every fishkeeping enthusiast. Plan on 20 gallons for the first fancy goldfish and 10 more for each additional one, so a 40-gallon tank holds two or three. Slim-bodied types of goldfish such as common and comet goldfish do better in a pond. |
| Three Spot Gourami | A fish that masterfully combines an unusual shape with its original appearance, the gourami is a fish that every owner of a 40-gallon tank should definitely consider. Keep a single fish or a pair, since males are territorial; 55 gallons or more is better if you want several. |
40 Gallon Tank Setup Ideas
A 40-gallon aquarium gives you a lot of creative freedom. Pick the fish and the style together, because the aquascape decides which fish will be comfortable.
One of the most popular choices is a community tank. It lets you keep many colorful fish of different sizes alongside live plants.
An example of a community stocking plan for a 40-gallon tank:
- 8 to 10 Neon Tetras (or another small schooling tetra; schools of 8 or more look and behave better)
- 5 Guppies (keep a ratio of one male to two or three females to limit harassment, and be ready for fry)
- 6 Zebra Danios (active; a group of 6 or more is better than a few)
- 1 Bristlenose Pleco
- 1 Dwarf Gourami (a single male can be bossy, so choose one carefully)
Fast, active fish like danios can stress shy or long-finned fish like guppies and gouramis. If you want a calmer tank, skip the danios.
Another option is a species-only tank, such as cichlids. The tank looks clean and tidy with fewer species. You could keep a group of Firemouth Cichlids, a group of Peacock Cichlids, or a group of Dwarf Cichlids. Cichlids need a stocking plan, since several males in one tank can fight. Check each species’ size and temperament before you buy.
You could also consider a paludarium, a tank that is part water and part land with terrestrial plants. It is less common but very distinctive. Species that suit a paludarium include Celestial Pearl Danios, Mollies, and Guppies. A paludarium needs a tight lid, so the fish cannot jump out.
40 Gallon Aquascape Ideas
A 40-gallon breeder is one of the best sizes for aquascaping. The 18-inch depth lets you build a foreground, midground and background, so the layout looks three-dimensional. Here are five styles that work well:
- Nature style: a few pieces of driftwood or stone, a mix of plants, and a natural, slightly asymmetrical look. This is the best starting point.
- Iwagumi: a minimalist layout built from a small number of stones (typically an odd number) with a carpet or short grass-like plants. It needs good lighting and careful stone placement.
- Dutch style: dense rows of different leaf shapes, colors and heights. It takes regular trimming.
- Jungle or low-tech: tall, fast-growing plants such as hornwort, vallisneria and Amazon swords with driftwood. It needs little equipment and hides filters well.
- Biotope: a tank copying a natural habitat, such as a blackwater stream with leaf litter and wood for tetras and rasboras.
Tips from experienced aquascapers:
- Place hardscape first. Position your largest rock or wood piece first, then build around it.
- Use the rule of thirds. Put the focal point at one of the intersections of a 3 x 3 grid, not in the center.
- Slope the substrate. Make it deeper at the back (about 3 inches) than the front (about 1 inch) to add depth.
- Plant in layers. Tall plants at the back, medium ones in the middle, short ones at the front.
- Start with easy plants. Anubias, java fern and cryptocoryne grow with low light and no CO2.
- Leave swimming space. Fill only part of the tank with hardscape and keep open water for the fish.
How to Setup a Fish Tank
Step 1: Tank and Equipment Setup
Place the tank on its stand and check that it is level. Rinse the substrate to wash out dust, then spread it across the base, building a slope toward the back if you want depth.
Next, install the equipment. Good circulation keeps the water healthy. The exact process depends on the filter and heater model.
Add the hardscape, then fill the tank slowly, pouring onto a plate or bag so you do not disturb the substrate. Add plants, treat the water with dechlorinator, and turn on the equipment.
Leave plenty of open space for the fish to swim. As a rough guide, keep a large part of the tank free of decoration.
Step 2: Cycling The Tank
Cycling builds the beneficial bacteria that break down toxic ammonia and nitrite into less harmful nitrate. Without it, ammonia can kill fish.
A fishless cycle usually takes about 4 to 6 weeks, though it can be faster with seeded media or plants, and sometimes slower. Add a source of ammonia, then use a test kit to watch ammonia and nitrite rise and then fall. When ammonia and nitrite both test at zero and you see nitrate, the tank is ready for fish. Do a large water change to bring nitrate down before adding fish.
Step 3: Adding Fish
Fish are often stressed when they arrive in the store because they have been transported some distance, so make the move to your tank gentle.
Float the bag in the tank for about 20 to 30 minutes so the temperature matches. Then add a little tank water to the bag every 5 to 10 minutes for 20 to 30 minutes, which helps the fish adjust to your water chemistry. Net the fish into the tank and discard the bag water.
Add fish in small groups over several weeks, not all at once, so the filter can keep up. Feed lightly for the first week.
Summary
The 40-gallon aquarium is a flexible, medium-sized tank that suits most community, planted and species-only setups, and it can hold many beautiful species. It is large enough to keep water stable and to give you a real aquascape, and small enough for most homes.
Pick the shape and aquascape style first, then choose the fish to match. Be patient with cycling and add stock gradually, and the tank will be easy to maintain.
Do you have a 40-gallon aquarium? Let us know in the comments section below…
We currently have a 40 gallon long and it houses 1 firemouth, 1 angel fish, 1 rainbow shark, a bristlenose and 3 tetras. Everyone gets along fairly well but they are not fully grown yet so time will tell. Our next tank that is nearly ready for inhabitants is a 55 gallon and we want to do African cichlids.
Overstocked I fear? 5 rainbow shiners, 5 pygmy loach 12 cloud minnows, 5 neon tetra, 4 zebra nano,1 bristle nose plec, 5 zebra tetra. In a well planted 50 gallon tank. I carry out a 20 gallon weekly water change,prune plants and clean impeller and sponge. There is a thriving colony of shrimp.
Morning Robert. I want to start breeding with show guppies. Can you perhaps give me some info how to start breeding them. Thanks Japie.
Hi Japie, thanks for your question. You’ll be able to find information on breeding Guppies here: http://fishkeepingworld.com/guppies/ Thanks, Robert
I have a 40gal long. All Cichlids ranging from about 3-7 in. I use a canister filter with a sponge filter for improved bacteria growth. I change the polishing filter weekly to maintain maximal flow through the filter. Airstones provide proper water movement. I chose Chiclids because my tap water is around pH 7.8-8.0
Hello Robert,
on this site when you refer to the volume of aquariums in gallons, what is it in litres? Are you using UK or US gallons?
Thanks!
Im looking to setup a 40 galloon tank and i want to know how long i should have my tank setup before actually introducing fish to the tank. I would also like to know the best schooling fish and any other fish that you recommend thatll be interesting to 6 yr olds.
Hi, I have a 40 gallon tank edits almost finished, being cycled and I want to add some fish. My mom really likes neon tetras and I’m planning on adding some snails. Are there any other fish that you guys recommend me adding, and how many of each should I add?
I want to have mainly neon tetras in my 40 gal. How many could i fit in a 40 gal comfortably, and what other small fish would go well with tetras?