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Choosing how many of each fish to keep is one of the most important decisions you will make for your aquarium. It affects water quality, fish health, behavior, and how your tank looks. New aquarists are often told a simple rule like one inch of fish per gallon, but that can cause problems. In this friendly guide, you will learn a better way to plan your stocking, easy numbers for the most popular fish, and clear examples for common tank sizes. By the end, you will feel confident choosing how many fish to keep, how to group them, and how to grow your community safely.
Why the number of fish matters
Fish release waste that turns into ammonia. Your filter converts ammonia into nitrite and then nitrate. If you keep too many fish or add them too fast, your filter cannot keep up, and ammonia and nitrite will rise. These are toxic and stress fish, which leads to disease. The right number of fish protects water quality and makes maintenance easier.
Stocking also affects behavior. Many species need a group to feel safe. Others defend territory and need space. A well-planned mix leads to natural behavior and calm fish. An overcrowded or mismatched tank leads to fighting, hiding, and injuries.
The simple rules that mislead
The one-inch-per-gallon rule and similar shortcuts sound helpful but ignore key facts. A slim neon tetra is not the same as a deep-bodied angelfish, and a six-inch pleco can make much more waste than six one-inch fish. Tank shape matters as well. A long tank has more swimming space and oxygen exchange than a tall tank with the same volume. Instead of a single number, use a practical step-by-step method.
A practical way to decide your stocking
Step 1: Learn adult size and behavior
Look up the adult size of each species. Always plan for the final size, not the store size. Check whether the fish is peaceful or territorial, whether it prefers groups or to be kept alone, and which part of the tank it uses. This helps you avoid crowding and choose good tankmates.
Step 2: Plan groups, pairs, or singles
Many fish do best in groups. Schooling and shoaling fish like tetras, rasboras, danios, and corydoras should be kept in groups of at least six, and eight to twelve is even better for small species. Some fish are best kept as a single, such as a male betta or a single dwarf gourami in smaller tanks. Some fish are best as a pair or small harem, like many dwarf cichlids or livebearers with one male to multiple females. Decide your minimum group sizes first, then check if the tank can support them.
Step 3: Check your tank footprint and filtration
The length and width of the tank matter more than height for swimming and territory. A 20-gallon long often suits active fish better than a 20-gallon tall. Filtration should turn the tank volume over four to ten times per hour, depending on the fish. Messy fish or crowded cichlid tanks need more flow and more filter media. Strong filtration does not replace proper stocking, but it gives you a safer buffer.
Step 4: Set a stocking budget using nitrates
Use water testing to set your safe stocking level. After the tank is fully cycled, test nitrate at the end of a normal week right before your water change. Try to keep nitrate under 20 to 40 ppm, and limit weekly increases to about 10 ppm. If nitrate rises faster, reduce feeding, increase water changes, upgrade filtration, or keep fewer fish. This method adapts to your real tank, not a generic rule.
Step 5: Add fish slowly and observe
Never add a large number at once. Add one group, wait two weeks, test water, and watch behavior. Then add the next group. This gives your filter time to grow beneficial bacteria and keeps stress low. If you see aggression or hiding, adjust the scape, add more hiding spots, or increase group size if it is a schooling species.
Stocking guidelines by fish type
Schooling and shoaling fish
Small schooling fish feel safe in numbers. Keep at least six of the same species, but eight to twelve looks better and reduces stress. Good beginner choices include neon tetras, ember tetras, black neon tetras, harlequin or chili rasboras, and zebra danios. In smaller tanks, pick one species and make a single larger school instead of many tiny groups. This looks more natural and makes the fish calmer.
Livebearers
Guppies, endlers, platies, mollies, and swordtails are fun and colorful. They breed easily. If keeping both sexes, plan for a ratio of one male to two or three females to spread out attention and reduce stress. Be ready to manage fry by giving them away or keeping only one sex. These fish prefer harder, more alkaline water. Mollies in particular like higher mineral content and can be messy eaters, so stock more lightly and maintain strong filtration.
Bottom dwellers
Corydoras catfish should be kept in groups of at least six of the same species. In a 20-gallon long or larger, eight to twelve small species like panda corys or pygmy corys do well. They need a soft sand or smooth fine gravel to protect their barbels. Loaches such as kuhli loaches are social and should be kept in groups of six or more in peaceful tanks with plenty of hiding spaces. Otocinclus catfish also prefer groups of six or more and need a mature tank with stable algae and biofilm. For plecos, choose the right species. A bristlenose pleco reaches around five inches and suits a 20- to 30-gallon tank, but a common pleco grows very large and needs a much bigger tank or a pond.
Gouramis and bettas
Honey gouramis are gentle and can be kept as a single, a pair, or a small group in a peaceful community. Dwarf gouramis are best kept as a single male in smaller tanks because males can fight. Pearl gouramis are larger and need at least a 30-gallon tank, often kept as one male with two females. Bettas are usually kept alone. A single male betta in a five- to ten-gallon tank with gentle filtration is ideal. Female betta sororities are challenging and not recommended for beginners because of hierarchy and aggression issues.
Cichlids
Dwarf cichlids like Apistogramma or rams can be kept as a pair or a male with two females, with plenty of hiding places and line-of-sight breaks. Kribensis can work as a pair in a 20-gallon long, but they become protective when breeding. Angelfish do best as a small group of juveniles in a larger tank to allow a pair to form, then keep a single pair in a 29-gallon or larger. Discus are sensitive and prefer to be in groups of six or more in a mature, warm, soft-water tank of 75 gallons or bigger. African cichlids from Lake Malawi often use an overstocking strategy to spread aggression. In a 55-gallon tank, keep about 15 to 20 juvenile mbuna with heavy filtration and frequent water changes, then remove extra males as they mature.
Goldfish
Fancy goldfish need space and produce lots of waste. A good rule is about 20 gallons for the first fancy goldfish and 10 to 20 gallons for each additional one, with strong filtration and regular water changes. Common and comet goldfish grow very large and are better suited to large tanks or ponds. Stock slowly, feed carefully, and be ready to upgrade filtration as they grow.
Shrimp and snails
Neocaridina shrimp like cherry shrimp can form a colony. Start with ten to twenty in a planted tank and let them grow. Amano shrimp are larger, less likely to reproduce in freshwater, and good in small groups of six to ten. Nerite snails are excellent algae grazers and do not breed in freshwater; one to three per 10 to 20 gallons is a common starting point. Remember that invertebrates also add to the bioload, though usually less than fish.
How many fish in common tank sizes
Five gallons
A five-gallon tank suits a single centerpiece or a nano group. A classic choice is one male betta with gentle filtration and a heater. Another option is a nano shoal like eight to twelve chili rasboras with plants and careful feeding. You can also keep a shrimp colony with ten to twenty cherry shrimp and a small snail or two. Avoid mixing several fish species in this size.
Ten gallons
In a ten-gallon community, choose one main school. For example, keep ten neon or ember tetras and add six dwarf corydoras or a small group of shrimp. Another option is a livebearer group of six to eight endlers or guppies of one sex to prevent breeding. A single honey gourami can be a peaceful centerpiece if the other fish are small and calm. Stay away from large or very active species like danios in this size unless the tank is long and open.
Twenty gallons
A 20-gallon long offers great flexibility. A balanced plan is twelve neon tetras, eight panda or julii corydoras, and one honey gourami. With plants and good filtration, this looks full without being crowded. Another option is a livebearer tank with two male and six female platies and a group of six corydoras. For a feature fish, keep a pair of Apistogramma with a small school of eight rasboras, but provide caves and line-of-sight breaks.
Twenty-nine to thirty gallons
This size can hold a larger school and a centerpiece. Try fifteen to twenty small tetras, eight to ten corydoras, and one to three honey or one dwarf gourami. For an angelfish plan, start with four to six juveniles and remove extras when a pair forms, leaving one pair with a small school of midwater fish that will not fit in their mouth. A shell-dweller cichlid colony is also possible with species-appropriate scaping and careful stocking.
Forty breeder
The 40-gallon breeder has a wide footprint, perfect for active fish and bottom dwellers. One example is sixteen to twenty harlequin rasboras, ten to twelve corydoras, a trio of honey gouramis, and a group of six amano shrimp. Another option is a rainbowfish tank with eight dwarf neon rainbowfish and a group of corydoras, or a livebearer display with one male and four female mollies plus a small cleanup crew, provided the water is hard and filtration strong.
Fifty-five gallons
A 55-gallon community can showcase a large, single school. Try twenty-five neon or black neon tetras, twelve corydoras, and a pair of dwarf cichlids such as rams or apistos. For African mbuna, stock around fifteen to twenty juveniles of three or four compatible species, provide many rock caves, and run heavy filtration with frequent water changes. Another path is goldfish, with two fancy goldfish and robust filtration, or a rainbowfish group of eight to ten medium species with matching water conditions.
Seventy-five gallons
In 75 gallons, you can keep larger or more complex groups. A discus plan would be six discus with a group of twelve rummynose tetras, ten corydoras sterbai, and strong heating and filtration. For South American community style, choose a single large school of thirty small tetras, twelve corydoras, a pair of apistos, and a bristlenose pleco. For angelfish lovers, keep a group of six juveniles and narrow down to a pair or trio as they mature, with a compatible midwater school and careful feeding.
Stocking example menus you can copy
Peaceful planted community
In a 20-gallon long, keep twelve ember tetras, eight panda corydoras, one honey gourami, and six amano shrimp. Use sand for the corys, add floating plants for the gourami, and run a gentle filter. This setup is active, colorful, and easy to maintain.
Nano aquascape focus
In a 10-gallon planted tank, keep ten chili rasboras and ten cherry shrimp. The rasboras will school in the open areas, and the shrimp will graze on plants and hardscape. This low-bioload plan allows slower growth of the biofilter and stable water for a new aquarist.
Hardwater livebearer garden
In a 29-gallon tank with hard, alkaline water, keep one male and four female mollies, eight to ten platies with mostly females, and ten to twelve amano shrimp. Feed carefully, trim plants, and plan how to rehome fry. Add extra filtration because livebearers eat and produce a lot.
Beginner African cichlid rock pile
In a 55-gallon lake Malawi mbuna tank, stock fifteen to twenty juveniles from three to four species with different colors to reduce cross aggression. Build a deep rock structure with many caves from bottom to top, run two strong filters, and change water weekly. As individuals mature, remove extra males to maintain balance. Overcrowding is intentional here to spread aggression, but you must maintain water quality.
Common questions and easy answers
Do algae eaters count toward stocking?
Yes. Plecos, otocinclus, and snails all produce waste. They help with specific types of algae or leftover food, but they do not replace regular cleaning. Choose species that fit your tank size and count them in your bioload.
Can I keep multiple small schools?
In small tanks, it is better to keep one larger school than several tiny groups. A single school of twelve looks more natural than three groups of four, and the fish feel safer. In larger tanks, two schools can work if each group has at least eight to twelve fish and you have the space.
How do I handle territorial fish?
Use hardscape and plants to create barriers and hiding spots. Break lines of sight so fish cannot chase each other across the entire tank. For species that defend a cave or area, provide more spots than you have territorial fish. In some cichlid setups, slightly higher numbers help spread aggression, but you must be strict about filtration and maintenance.
What about mixing temperatures and water chemistry?
Choose fish that like similar temperatures and water hardness. Tetras and corydoras usually prefer warmer, softer water. Livebearers do better in harder, more alkaline water. Research your tap water and pick fish that match, or use methods to adjust the water if you have experience. Stocking the right numbers will not help if the water itself is unsuitable.
How fast can I add fish?
After your tank is fully cycled, add one group at a time and wait at least one to two weeks before adding the next group. Test ammonia, nitrite, and nitrate. If ammonia or nitrite show up, pause and let the filter catch up. Slow, steady additions protect your fish.
Fine-tuning your plan with filtration and maintenance
Match your filter to your fish. For small communities, aim for four to six times the tank volume per hour. For messy fish, large schools, or cichlids, aim for six to ten times with extra media. Rinse filter media in tank water during water changes to protect beneficial bacteria. Do not replace all media at once.
Feed small amounts one to two times per day so fish finish food in a minute or two. Overfeeding causes more waste and higher nitrates. Do weekly water changes of 25 to 50 percent, or more if your nitrate rises quickly. Keep a simple test kit and check nitrate regularly. If nitrate increases by more than about 10 ppm per week, reduce feeding, increase plant mass, change more water, or lighten your stocking.
Plants help by using nitrate, but do not rely on them alone. Good flow, oxygenation, and clean substrates matter. Vacuum gently around plants and decor to remove trapped waste. For bottom dwellers, use smooth sand or fine gravel. For species that breathe air like bettas and gouramis, ensure access to the surface without strong current.
Quick numbers for popular fish
Neon or ember tetras do best in groups of eight to fifteen. Harlequin rasboras are happy in groups of eight to twelve. Zebra danios should be kept in groups of six or more and prefer long tanks. Corydoras should be kept in groups of six or more, with eight to twelve for small species in larger tanks. Otocinclus need groups of six or more in mature tanks. Honey gouramis can be kept as one, a pair, or a small group; dwarf gourami as a single male in smaller tanks. A male betta is best kept alone. Guppies, endlers, platies, and mollies do well at one male to two or three females if breeding is planned; otherwise keep a single sex. Bristlenose pleco is usually one per small to medium tank. Angelfish form pairs; keep one pair in 29 gallons or more. Discus need groups of six or more and large, warm tanks. Fancy goldfish need at least 20 gallons for the first and 10 to 20 gallons for each additional fish, with heavy filtration.
Putting it all together
Start with your tank’s size and shape. Choose fish that match your water and experience level. Decide which species require groups, which are singles, and which are pairs. Plan around the adult size, not the store size. Build your scape to fit the fish, with open swimming space for schoolers, bottom area for corydoras, caves for cichlids, and quiet corners for labyrinth fish like gouramis and bettas. Add fish slowly, test water weekly, and adjust based on nitrate and behavior. If you see stress, reduce stocking, increase water changes, or change the mix.
Conclusion
There is no single number that fits every aquarium. The best answer to how many of each fish you should keep blends biology, behavior, and your maintenance routine. Keep schooling fish in proper groups, give territorial fish room and structure, and match species to your water. Use filtration and water changes to support your plan, and let nitrate be your guide to safe stocking. Start with conservative numbers, add fish slowly, and enjoy watching a calm, balanced community. With these simple steps, you can build a beautiful aquarium where every fish has the space and company it needs to thrive.