Best Fish for Small Tanks and Nano Aquariums

Small tanks and nano aquariums can be beautiful, peaceful, and surprisingly easy to care for when you choose the right fish. The key is matching species to the limited space and water volume. In this guide, you will learn how to pick the best fish for small tanks, understand how many you can keep, and discover simple ways to set up and care for a thriving nano community. Whether you have a 2.5, 5, 10, or 20 gallon aquarium, there are great options that fit your style and experience level.

What Is a Nano Aquarium and Why It Matters

A nano aquarium is usually any freshwater tank under 20 gallons. Common sizes include 2.5, 3, 4, 5, 7.5, 9, and 10 gallons, as well as 15 and 20 gallons. The small footprint makes them perfect for desks, nightstands, or small rooms. However, less water means faster changes in temperature and water chemistry. This is why choosing the right fish is essential for success. Some fish are small but very active, some are shy and need plants, and some need to live in groups. A good fit leads to happier fish and easier maintenance for you.

Core Principles for Stocking Small Tanks

Size and Activity Over Body Length

Do not choose fish by size alone. A small fish can still need a lot of space if it is fast or constantly swimming. For example, a zebra danio is short but highly active and better suited to a long tank of 20 gallons or more. On the other hand, a betta is similar in length but moves slowly and does well in 5 gallons or larger when kept alone.

Bioload and Filtration

Bioload is how much waste your fish and food add to the water. Small tanks cannot process large amounts of waste. Use a gentle, reliable filter, and avoid overfeeding. Sponge filters are popular in nanos because they offer excellent biological filtration and soft water flow that will not toss your fish around.

Water Stability and Routine

In a nano tank, ammonia and nitrite spikes can happen quickly. Cycle your tank before adding fish by growing beneficial bacteria in the filter. After the tank is cycled, do regular partial water changes, usually 20 to 40 percent each week, to keep nitrate low and pH steady. Consistency is the secret to success.

Social Needs and Group Sizes

Many tiny fish are schooling or shoaling species that feel safe in groups. It is better to keep a proper group of one species than to mix many singles that feel stressed. For example, ember tetras are most confident in groups of eight or more, and pygmy corydoras do best in a group of at least eight. Always plan your stock around the group needs of your chosen species.

Tank Shape and Horizontal Space

Long tanks are generally better than tall tanks because most fish swim horizontally. A 10 gallon tank that is longer gives more usable swimming space than a cube of the same volume. For bottom dwellers like pygmy corydoras, a larger footprint with soft substrate increases comfort and natural behavior.

Aquascaping for Comfort

Plants, driftwood, caves, and rocks break up sightlines and provide cover. When fish feel secure, they show brighter colors and natural behavior. Floating plants like frogbit or salvinia help shy species; ground plants like crypts and moss provide grazing surfaces for shrimp and microfauna that support nano fish diets.

Let the Tank Mature

Some species, especially otocinclus and delicate rasboras, do better in a mature tank with stable biofilm and algae. If you can wait six to eight weeks after cycling before adding sensitive fish, you will see higher success and less stress.

Best Fish for 2.5 to 5 Gallon Tanks

The smaller the tank, the more selective you must be. Stock lightly, choose a single centerpiece fish or a very small group, and rely on plants to fill the space with life.

Betta Splendens

Bettas are classic nano fish because they are slow swimmers and breathe air at the surface. A single male betta or a calm female is ideal for a 5 gallon tank with a heater and a gentle filter. They prefer warm water between 76 and 80°F, calm flow, and dense planting. Avoid tankmates in very small tanks; shrimp may be hunted, and fast fish may nip fins or stress the betta. Choose a wide, stable tank rather than a bowl, and provide a lid because bettas can jump.

Chili Rasbora Boraras brigittae

Chili rasboras are tiny, bright red fish that thrive in soft, slightly acidic to neutral water with heavy plants and low to moderate light. In 5 gallons, a group of 8 to 12 is possible if filtration is gentle and maintenance is consistent. They show best color with dark substrate and floating plants. Keep the tank mature and feed micro foods like crushed flakes, baby brine shrimp, or micro pellets. They are very peaceful and do best without larger or pushy tankmates.

Sparkling Gourami Trichopsis pumila

Sparkling gouramis are small, shy labyrinth fish that produce soft croaking sounds during courtship. They enjoy calm, warm water and dense planting. In 5 gallons, keep a single fish with no other midwater species for a simple, relaxing display. In 10 gallons or more, a pair or trio works better. Provide floating plants and gentle filtration. Because they are timid, avoid fast feeders that will outcompete them at meal time.

Scarlet Badis Dario dario

Scarlet badis are jewel-toned micro predators best kept as a single male in 5 gallons or a pair in a larger, heavily planted tank. They prefer cool to mid 70s Fahrenheit and need tiny live or frozen foods such as daphnia, baby brine shrimp, or microworms. They can refuse dry food, so they are better for owners willing to provide small live foods. Keep them without fast competitors and offer plenty of hiding spots.

Endler’s Livebearer Poecilia wingei

Endlers are hardy, colorful, and active. They do well in 5 gallons as a small, all-male group of three to five fish. Mixed sexes will breed very quickly and overwhelm a nano tank, so start with males only unless you have a plan for fry. Provide clean water, stable temperature around 74 to 78°F, and regular water changes. They accept a wide range of foods and are excellent for beginners who prefer movement and sparkle.

Shrimp and Snails for Nano Tanks

While not fish, neocaridina shrimp and nerite snails are often the best choices for 2.5 to 5 gallons. Shrimp thrive in stable, planted tanks with sponge filters and grazing surfaces. Nerite snails help with algae and do not reproduce in freshwater. If you want a very low bioload display, a shrimp colony with a single small snail is simple, active, and beautiful.

Best Fish for 10 Gallon Nano Aquariums

A 10 gallon tank opens many more options. You can keep modest groups of micro fish and even mix one centerpiece fish with a small schooling species if you manage bioload and provide enough plants and hiding places.

Celestial Pearl Danio Danio margaritatus

Also known as the galaxy rasbora, this species shines in planted tanks with gentle flow. A group of 8 to 12 in 10 gallons is a great start. They are shy at first but become bolder with cover, a dark substrate, and soft lighting. Keep them at 72 to 78°F and feed small, high quality foods. They are peaceful and do well with pygmy corydoras and shrimp, provided there is enough cover for shrimplets.

Ember Tetra Hyphessobrycon amandae

Ember tetras glow orange and prefer groups of 10 to 15 in a planted 10 gallon tank. While some experienced aquarists keep a small group in 5 gallons, 10 gallons gives them more room to school and explore. They like warmer water around 76 to 80°F and calm tankmates. With good feeding and stable water, their color deepens and they become very active.

Pygmy Corydoras Corydoras pygmaeus and Hastatus/Habrosus group

Pygmy corys are gentle bottom dwellers that also school midwater. They are happiest in groups of 8 to 12. Use soft sand or very smooth gravel to protect their barbels and provide leaf litter, wood, and plants for cover. Keep temperatures in the mid 70s and keep the water clean and oxygenated. They pair well with small rasboras and tetras and make a lively, peaceful display.

Honey Gourami Trichogaster chuna

Honey gouramis are calm, colorful, and smaller than most gourami species. A single honey gourami can live in a well scaped 10 gallon with a small school of micro fish, as long as the tank is not crowded and there are plants for cover. They prefer 76 to 80°F, gentle flow, and peaceful companions. A pair is better in a 15 to 20 gallon tank.

Least Killifish Heterandria formosa

Despite the name, they are livebearers, not true killifish. They are among the smallest vertebrates and are perfect for heavily planted 10 gallon tanks. They tolerate a range of water hardness and do best in cooler tropical temperatures around 72 to 78°F. They reproduce steadily but not explosively. Dense plants will protect fry, so plan populations by adjusting plant density and feeding.

Clown Killifish Epiplatys annulatus

Clown killifish are surface-dwelling beauties with striking striping and a lyretail. They prefer calm water, tight lids, and floating plants. In a 10 gallon, keep a pair or trio with small, peaceful midwater fish that will not outcompete them for food. They appreciate soft water but adapt well over time. Because they jump, a lid is essential.

Pea Puffer Carinotetraodon travancoricus

A single pea puffer can live in a heavily planted 10 gallon species tank. They are intelligent, curious, and great for advanced beginners who can meet their feeding needs. They prefer live or frozen foods such as snails, bloodworms, and brine shrimp. Do not keep them with long finned or slow fish; they nip. Provide visual barriers so they can explore without stress. A snail culture is helpful to keep their teeth healthy.

Otocinclus Catfish Otocinclus spp.

Otocinclus are gentle algae grazers and do best in mature tanks with stable parameters and plenty of biofilm. In 10 gallons, a group of 4 to 6 may work if the tank is well established, planted, and lightly stocked. Many keepers find them easier in 15 gallons or more. Supplement their diet with blanched vegetables and quality algae wafers once the natural algae runs low.

Best Fish for 15 to 20 Gallon Nanos

These tanks are still considered nano but offer far more stability and room for groups. You can build classic small schools and even include a careful centerpiece fish in a peaceful community.

Harlequin and Lambchop Rasbora Trigonostigma species

These rasboras are calm, elegant swimmers that shine in groups of 10 to 15. A 20 gallon long tank gives them space to school. They adapt to a wide range of water hardness and do well at 74 to 78°F. Include plants, open swimming areas, and a dark background for best color.

Neon and Green Neon Tetra Paracheirodon species

Neon tetras and their close relatives enjoy slightly softer, acidic to neutral water, dense plants, and dappled light. Keep 12 or more in a 20 gallon planted tank for a glowing school effect. Stable water is more important than chasing the perfect pH. They pair nicely with small corydoras and peaceful centerpiece fish like a single honey gourami.

Dwarf Pencilfish Nannostomus marginatus and related species

Dwarf pencilfish are quiet, surface to midwater fish that appreciate calm flow, floating plants, and gentle tankmates. A group of 10 to 12 in a 15 or 20 gallon tank creates a soft, natural look. They enjoy micro foods and are a great option if you prefer a delicate, understated display.

Kuhli Loach Pangio kuhlii group

Kuhli loaches are eel like bottom dwellers that need soft sand and many hiding places. They prefer to live in groups of at least 6 to 10. A 20 gallon long tank is the minimum for a comfortable group. Keep the tank covered because they explore at night and can escape. They are peaceful and pair well with small rasboras and tetras.

Apistogramma Dwarf Cichlids

In a mature, well scaped 20 gallon long, a single pair of a peaceful Apistogramma species can be a stunning centerpiece. Provide caves, leaf litter, and visual barriers, and avoid aggressive tankmates. They are best for keepers willing to maintain stable water and observe behavior closely. If you are brand new to the hobby, you may want to start with easier species first.

Beginner Friendly Stocking Ideas by Tank Size

Simple 5 Gallon Plans

For a peaceful, low stress setup, choose one centerpiece species and build the scape around it. A single betta in a planted 5 gallon with a sponge filter is classic and easy. Alternatively, keep a group of 8 to 10 chili rasboras in a blackwater style aquarium with floating plants and gentle flow. If you want almost no fish at all, consider a shrimp colony with a nerite snail; the shrimp will be very active and the snail helps control algae.

Balanced 10 Gallon Plans

A planted 10 gallon can hold a small school plus a bottom group. A good example is 10 ember tetras with 8 pygmy corydoras. Another nice community is 10 celestial pearl danios with a single honey gourami as a centerpiece. If you love personality and a species only approach, keep one pea puffer with dense plants and lots of hardscape to explore.

Active 20 Gallon Plans

In a 20 gallon long, you can keep larger groups for natural behavior. Try 12 to 15 harlequin rasboras with 10 pygmy corydoras and a single honey gourami. Another option is a neon tetra school with a small group of kuhli loaches. Keep the aquascape open in the middle for schooling and thicker at the sides and back for shelter.

Feeding Micro Fish the Right Way

Match Food to Mouth Size

Nano fish often have tiny mouths and do best with small foods. Use crushed flakes, micro pellets, or powdered foods designed for small fish. For variety and color, offer frozen or live baby brine shrimp, daphnia, and cyclops. Avoid large pellets that sink untouched and foul the water.

Routine and Moderation

Feed small amounts one to two times per day. In a nano tank, it is safer to slightly underfeed than overfeed. Watch your fish to make sure everyone gets a bite, and remove uneaten food after a few minutes. Specialized feeders like scarlet badis may need targeted feeding with a pipette to ensure they eat.

Support Grazers and Biofilm

Otocinclus, shrimp, and some rasboras enjoy biofilm and soft algae that grow on wood, rocks, and plant leaves. Do not scrub every surface perfectly clean. Instead, allow some gentle growth on hardscape and supplement with algae wafers, blanched zucchini, or spinach for grazing species.

Plants and Equipment for Small Tanks

Filters That Fit Nanos

Sponge filters are ideal for nanos. They provide strong biological filtration with gentle flow, are safe for shrimp, and are easy to clean. A small hang on back filter also works if you baffle the outflow to avoid blasting your fish. In all cases, keep maintenance simple and consistent.

Heaters and Temperature Stability

Most nano fish are tropical and need stable temperatures between 74 and 80°F. Use a reliable adjustable heater sized for your tank. In small volumes, room temperature shifts can be dramatic, so stability matters. A lid helps reduce evaporation and heat loss, and it also prevents jumping.

Lighting and Easy Plants

Choose gentle lighting so fish do not feel exposed. Easy, low maintenance plants include java fern, anubias, mosses, crypts, and floating plants like salvinia and frogbit. Plants improve water quality by using up nitrates and provide shelter, grazing surfaces, and shade that many nano fish appreciate.

Substrate and Hardscape

Sand or fine gravel is comfortable for small bottom dwellers. If you keep pygmy corydoras or kuhli loaches, soft sand is best to protect barbels and encourage natural behavior. Add driftwood, small stones, and leaf litter to create a natural look and microhabitats for your fish.

Water Care and Simple Maintenance

Weekly Water Changes

Plan to change 20 to 40 percent of the water each week, depending on stock and feeding. In a 5 gallon tank, smaller but more frequent water changes can prevent swings. Always match temperature between new and old water and add a conditioner that removes chlorine and chloramine.

Testing and Parameters

Use a basic test kit to track ammonia, nitrite, nitrate, and pH. Ammonia and nitrite should be zero. Nitrate should ideally stay below 20 to 30 ppm. Most nano fish adapt to a range of pH as long as it stays stable. If you keep shrimp, pay attention to hardness and minerals; they need some calcium and magnesium for healthy molts.

Cleaning Without Disruption

Gently squeeze sponge filters in a bucket of tank water during changes, not under tap water, to preserve beneficial bacteria. Vacuum lightly and avoid overcleaning planted areas where biofilm and microfauna support your fish. Clean glass and remove only excess debris; leave beneficial surfaces intact.

Compatibility and Behavior Tips

Watch for Fin Nipping

Some tiny fish will nip if crowded or bored. Avoid mixing nippy species with long fin fish like male bettas. Keep schools large enough to focus their energy on each other and provide a well structured scape to break lines of sight.

Territorial or Shy Species

Centerpiece fish like bettas, honey gouramis, or pea puffers often prefer to be the only midwater star in a small tank. If you want companions, choose calm species that occupy a different level, like pygmy corydoras. For shy fish such as celestial pearl danios, add more plants and overhead cover to build confidence.

Livebearers and Population Control

Guppies and endlers reproduce quickly. In small tanks, stick to all male groups or have a clear plan to rehome fry. Heavy planting will help, but it will not stop population growth. If you prefer a stable, predictable group, choose schooling species that do not breed as often in community tanks.

Common Mistakes to Avoid

Overstocking is the number one error in nano tanks. Always plan for adult size and activity, not just current size. Skipping the nitrogen cycle leads to dangerous ammonia spikes. Be patient and cycle before adding fish. Overfeeding is another common issue; fish need less food than you think, especially in small tanks. Avoid mixing too many species in a tiny space. It is better to see natural group behavior from one or two species than to have a chaotic mix. Finally, rushing to add delicate fish like otocinclus or scarlet badis to a new tank can lead to losses. Wait for a mature system before adding sensitive species.

How to Choose and Buy Healthy Nano Fish

Look for fish that are alert, active, and eating at the store. Their fins should be intact, colors bright, and bodies free of spots, fuzz, or wounds. Avoid tanks with sick fish nearby because diseases often spread through shared nets or water. Ask for fish from tanks with similar water to yours if possible. When you bring fish home, float the bag to match temperature, then gradually add small amounts of your tank water before release. If you can set up a small quarantine tank for two to four weeks, you will protect your main display from parasites and illness.

Mini Profiles with Practical Parameters

Betta Splendens

Best kept alone in 5 gallons or more. Temperature 76 to 80°F, calm flow, plenty of plants. Accepts pellets, flakes, and frozen foods. Avoid fin nippers and fast tankmates.

Chili Rasbora

Group of 8 to 12 in 5 to 10 gallons. Temperature 74 to 80°F. Soft, shaded, planted tank. Eats micro foods. Very peaceful and sensitive to poor water quality.

Ember Tetra

Group of 10 to 15 in 10 gallons or more. Temperature 76 to 80°F. Planted, gentle flow, dark substrate shows color. Micro foods and small pellets.

Celestial Pearl Danio

Group of 8 to 12 in 10 gallons. Temperature 72 to 78°F. Dense plants and cover, low to moderate light. Mixed diet of micro foods and small frozen items.

Pygmy Corydoras

Group of 8 to 12 in 10 gallons or more. Temperature 72 to 78°F. Soft sand, clean water, oxygenation. Eats sinking micro pellets and small frozen foods.

Honey Gourami

Single in 10 gallons or pair in 15 to 20 gallons. Temperature 76 to 80°F. Calm, planted tanks. Accepts a wide diet. Avoid boisterous tankmates.

Pea Puffer

Single in 10 gallons species tank. Temperature 76 to 80°F. Heavily planted with line of sight breaks. Needs live or frozen foods and snails.

Clown Killifish

Pair or trio in 10 gallons. Temperature 74 to 80°F. Floating plants and tight lid. Eats small floating foods and tiny live prey.

Endler’s Livebearer

All male group of 3 to 6 in 5 to 10 gallons. Temperature 74 to 78°F. Hardy and active. Will breed fast if sexes are mixed.

Otocinclus

Group of 4 to 6 in mature 10 to 20 gallon tanks. Temperature 72 to 78°F. Needs biofilm and algae, plus supplemental foods. Sensitive to new tanks.

Planning Your First Nano Setup Step by Step

Choose the Tank and Stand

Pick the largest tank your space allows. A 10 or 20 gallon is more forgiving than a 5 gallon and gives you better fish choices. Place it on a level, sturdy surface away from direct sun and heat sources.

Add Filter, Heater, and Hardscape

Install a sponge filter or a gentle hang on back filter, set the heater to your target range, and place driftwood and rocks to create zones and sightline breaks. Leave open areas for swimming and pockets of cover for resting.

Plant Heavily and Cycle

Add easy plants, root tabs if needed, and start cycling. Use bottled bacteria if you want to speed the process. Test water until ammonia and nitrite read zero for a week in a row. Only then add fish slowly, monitoring parameters along the way.

Stock Slowly and Observe

Introduce one group or centerpiece at a time. Watch for stress, eating behavior, and compatibility. Adjust feeding and flow if fish hide constantly or gasp at the surface. Patience at this stage prevents problems later.

Troubleshooting Common Nano Issues

Cloudy Water

Cloudiness often comes from bacterial blooms or overfeeding. Reduce feeding, increase water changes, and give your filter time to mature. Do not throw away your filter media; gently rinse it in tank water to keep bacteria alive.

Algae Growth

Small tanks get algae quickly if light is too strong or nutrients are high. Reduce light duration to six to eight hours, feed less, and add easy growers like floating plants to absorb excess nutrients. A nerite snail can help manage algae in tanks 5 gallons and up.

Shy or Faded Fish

Shy fish brighten and become bolder with more plants, darker substrate, and floating cover. Increase group sizes for schooling species. Check that water parameters are stable and that flow is not too strong.

Ethical and Long Term Considerations

Even small fish deserve space, stable water, and a safe environment. Avoid the temptation to keep many species in a tiny tank. Plan for adult size, long life spans, and the possibility of breeding. Rehome responsibly if populations grow. The most beautiful nano tanks are not crowded; they show natural schooling, calm behavior, and bright color because the fish feel secure.

Conclusion

Nano aquariums are a wonderful way to enjoy the world of fishkeeping in a small space. The secret is selecting fish that truly suit the volume and shape of your tank, then building a simple, stable environment around their needs. In 5 gallons, a single betta, a small group of chili rasboras, or a shrimp colony are excellent choices. At 10 gallons, you can enjoy ember tetras, celestial pearl danios, pygmy corydoras, and even a single honey gourami or pea puffer in the right setup. With 15 to 20 gallons, classic schools of rasboras, neons, and small bottom dwellers come to life. Keep stocking modest, feed tiny foods, add lots of plants, and change water regularly. Do these things, and your small tank will be vibrant, healthy, and easy to love for years to come.