20 Gallon Tank Stocking Ideas: 7 Complete Combinations
A 20 gallon tank is the first size where you can build a real community: two or three groups at different levels, with room to spare. Here are seven combinations that work.
In This Guide
Why 20 Gallons Is the Sweet Spot
Twenty gallons (76 litres) is where fishkeeping gets easier rather than harder. The extra water dilutes mistakes, the temperature is more stable, and the footprint — typically 60 × 30 cm — is finally enough for a proper school plus a bottom group.
- •Water parameters move slowly, so a missed water change is not a crisis.
- •There is room for two or three distinct groups instead of one.
- •Most dwarf cichlids become possible for the first time.
- •Standard 20 gallon tanks, hoods, lights and filters are cheap and widely available.
Ground Rules for All Seven
Each plan below assumes the tank is fully cycled, filtered, and gets a 25% water change weekly. Stock one group at a time, two weeks apart.
Check the temperature column
Every plan keeps all its species inside one temperature and pH window. Swapping a species between plans usually breaks that — check the ranges on each species page first.
The Seven Stocking Plans
1. The classic community
20 gal / 76 L · 24-26°C · pH 6.5-7.5- •8× Harlequin Rasbora (2 inches)
- •6× Bronze Corydoras (2.5 inches)
- •1× Honey Gourami (2 inches)
Three levels, three temperaments, no overlap. The gourami holds the surface, the rasboras the middle, the corydoras the floor. The most forgiving plan here.
2. Soft-water blackwater
20 gal / 76 L · 25-27°C · pH 5.5-7.0- •10× Cardinal Tetra (2 inches)
- •6× Sterbai Corydoras (2.5 inches)
- •1× Borelli's Dwarf Cichlid (2.5 inches)
Built for naturally soft water, with driftwood and leaf litter. The dwarf cichlid pairs off and claims a cave; the cardinals stay high and out of its way.
3. Hard-water livebearers
20 gal / 76 L · 24-26°C · pH 7.5-8.5- •6× Guppy (2 inches)
- •4× Platy (2.5 inches)
- •4× Nerite Snail (1 inch)
If your tap water is hard, this is the plan that stops fighting it. Keep two or three females per male to spread out breeding attention — and expect fry.
4. Cool water, no heater
20 gal / 76 L · 18-22°C · pH 6.5-7.5- •10× White Cloud Mountain Minnow (1.5 inches)
- •6× Peppered Corydoras (2.5 inches)
- •1× Hillstream Loach (3 inches)
A genuinely unheated tank for a cool room. All three species are subtropical and are shortened by tropical temperatures rather than helped by them.
5. The planted nano school
20 gal / 76 L · 24-26°C · pH 6.0-7.5- •12× Chili Rasbora (0.75 inches)
- •8× Cherry Shrimp (1 inch)
- •6× Otocinclus (2 inches)
Everything here is under 5 cm, which makes the tank look far larger than it is. Needs dense planting and an established tank — otocinclus starve in a new one.
6. Centrepiece cichlid pair
20 gal / 76 L · 25-27°C · pH 6.0-7.5- •2× Kribensis (4 inches)
- •8× Lemon Tetra (2 inches)
A kribensis pair is the most rewarding fish you can keep in this size. The tetras are dither fish — their calm swimming tells the cichlids no predator is around. Expect the pair to claim the bottom when breeding.
7. Species-only betta community
20 gal / 76 L · 25-27°C · pH 6.5-7.5- •1× Betta Fish (3 inches)
- •8× Lambchop Rasbora (1.5 inches)
- •6× Pygmy Corydoras (1 inch)
A betta in 20 gallons has room to avoid tankmates, which is what makes this work where a 5 gallon version does not. Choose a short-finned betta and avoid anything brightly coloured or long-finned.
Counts are minimums, not maximums
Where a plan says eight, eight is the smallest group that keeps the fish comfortable. Adding a few more of the same species is almost always better than adding a different species.
What Not to Put in a 20 Gallon
These are the species most often sold for tanks this size and most often outgrow them.
| Species | Minimum tank | Adult size | Temperature | pH | Group size |
|---|---|---|---|---|---|
| Common Pleco | 125 gal / 473 L | 18 inches / 46 cm | 72-86°F / 22-30°C | pH 6.5-7.5 | — |
| Bala Shark | 125 gal / 473 L | 14 inches / 36 cm | 72-82°F / 22-28°C | pH 6.5-8.0 | 5+ |
| Clown Loach | 100 gal / 378 L | 12 inches / 30 cm | 77-86°F / 25-30°C | pH 6.0-7.5 | 5+ |
| Oscar | 75 gal / 284 L | 14 inches / 36 cm | 74-81°F / 23-27°C | pH 6.0-8.0 | — |
| Tinfoil Barb | 100 gal / 378 L | 14 inches / 36 cm | 72-77°F / 22-25°C | pH 6.5-7.5 | 5+ |
| Red Tail Shark | 55 gal / 208 L | 6 inches / 15 cm | 72-79°F / 22-26°C | pH 6.5-7.5 | — |
Every one of these is commonly sold at 5-8 cm. The minimum tank column is what they actually need as adults.
Goldfish are not a 20 gallon fish either
A single fancy goldfish needs about 75 litres to itself and produces far more waste than any tropical fish of the same length. Common goldfish belong in ponds.
Frequently Asked Questions
How many fish can a 20 gallon tank hold?
Typically two or three groups — for example eight small mid-water schooling fish, six bottom-dwelling corydoras and one centrepiece fish, around 15 fish in total. The limit depends on adult size and waste output, not fish count alone.
What is the best community setup for a 20 gallon tank?
Eight harlequin rasboras, six bronze corydoras and one honey gourami. The three species occupy different levels, share the same water parameters and are all peaceful and beginner-friendly.
Can I keep a betta in a 20 gallon community tank?
Yes, and it works better than a smaller tank because the betta can avoid its tankmates. Choose a short-finned betta, keep it with small peaceful species such as rasboras and corydoras, and avoid brightly coloured or long-finned fish it might mistake for a rival.
Do I need a heater for a 20 gallon tank?
For most tropical species, yes. The cool-water plan above is the exception: white clouds, peppered corydoras and hillstream loaches are subtropical and do best at 18-22°C without a heater, provided the room stays in that range.
Build your own 20 gallon plan
Pick the species you like and check them against each other before you buy.