Guide
Good setups for a 125 litre tank
125 litres is about 33 US gallons. Most tanks at this size are tall rather than long - which changes what you should stock. Check combinations with our stocking calculator.
- Read the footprint, not the volume: the common 125 litre (Juwel Rio 125) is 81 × 36 cm - the same length as many 80 litre tanks. The extra 45 litres is height.
- That height unlocks angelfish, which an 80 litre cannot house. It does nothing for corydoras or fast horizontal swimmers.
- Three setups work: an angelfish pair with careful tank mates, a two-school community with a bristlenose, or a South American biotope.
- You can run two schools here - roughly 12 of one species and 10 of another - which is the first size where that looks right rather than crowded.
- Cardinals and adult angelfish are a risk: angels eat what fits in their mouths. Use rummynose or lemon tetras instead.
- Roughly 105-110 litres of real water once substrate and hardscape are in.
Why the footprint matters more than the number
This is the single most useful thing to understand about a 125 litre, and almost no stocking guide mentions it. The most common tank at this volume - the Juwel Rio 125 - measures 81 × 36 × 50 cm. Compare that to a typical 80 litre at 80 × 35 × 40 cm. The length is effectively identical. The entire difference is 10 cm of extra height.
Height is valuable for fish with tall bodies: angelfish, pearl gouramis, larger tetras that use the whole water column. It is close to worthless for corydoras, loaches, and anything that swims fast along a horizontal line. So a 125 litre is not "an 80 litre with more room for everything" - it is an 80 litre with a new category of fish unlocked.
A Juwel Rio 125 is 81 cm long. The extra volume over an 80 litre is vertical, so stock it for height-using species rather than assuming everything scales up.
If your priority is a big shoal of small fast fish, a long 100 litre will serve you better than a tall 125. If you want angelfish, gouramis, or a tank with visual depth from top to bottom, the 125 is the better buy. Decide which you want before you buy the tank, because the footprint is the one thing you cannot change afterwards.
Setup 1: an angelfish pair
A 125 litre with 50 cm of height is a realistic home for a pair of Pterophyllum scalare. Not six angelfish - a pair. Angels are cichlids, they pair off, and a bonded pair will claim the tank and harass the others.
Getting to a pair
Angelfish are not reliably sexable as juveniles, so the usual route is to grow out a group of five or six in a larger tank and rehome the losers once a pair forms. If that is not practical, buy an established bonded pair from a breeder. Buying two random juveniles and hoping is how most people end up with one dead angelfish.
Tank mates that survive
Adult angelfish eat what fits in their mouths, and neon and cardinal tetras fit. Use Hemigrammus rhodostomus (rummynose tetras) or Hyphessobrycon pulchripinnis (lemon tetras) instead - both are large enough to be safe and tight-shoaling enough to look good. Add eight Corydoras sterbai for the bottom, which handle the 26-28°C angelfish temperature better than most corydoras.
Do not keep angelfish with fin-nipping species. Serpae tetras, tiger barbs and Buenos Aires tetras will shred angelfish fins, and an angel cannot outswim them in a 36 cm-deep footprint. This is the most common cause of a "mystery" angelfish illness - the fin damage comes first, the infection follows.
Setup 2: the two-school community
125 litres is the first size where two distinct schools look deliberate rather than crowded. Twelve Paracheirodon axelrodi (cardinal tetras) in the mid-water, ten Trigonostigma heteromorpha (harlequin rasboras) slightly higher, eight Corydoras on the bottom, and one Ancistrus sp. (bristlenose pleco).
The trick to making two schools read as two schools is picking species that occupy different levels or have contrasting colour and movement. Cardinals hold a loose mid-water cloud; harlequins move as a tighter unit. Two species of similar tetra will simply merge into one indistinct group.
Target pH 6.5-7.2, 24-27°C, hardness 3-12 dGH. This is a soft-water community - do not add livebearers to it, because mollies and platies want harder, more alkaline water and one group will always be compromised.
Adding a second school before the first one is fully grown and settled. Stock one school, let the tank stabilise for four to six weeks, then add the next. Adding twenty fish at once to a newly cycled 125 litre will outpace your bacterial colony and produce an ammonia spike in a tank that looked finished.
Setup 3: a South American biotope
A quieter, more coherent option. A pair of Mikrogeophagus altispinosus (Bolivian rams), fifteen rummynose tetras, and eight Corydoras sterbai, over sand with driftwood, leaf litter and a few tannins in the water.
Botanicals - catappa leaves, alder cones - lower the pH gently, tint the water, and provide grazing biofilm for fry. The tea-coloured water looks unusual on day one and entirely right by week three. Our guide to tannins covers how far to take it.
A nominal 125 litre holds around 105-110 litres once substrate, wood and equipment are in. Dose medications and water conditioners against the real number, not the label.
What to avoid in a 125 litre
Discus: a group of five or six needs 200 litres minimum, very warm water at 28-30°C, and water changes far beyond a normal community routine. 125 litres is not enough for a group, and discus kept singly do badly.
Six angelfish: covered above, but worth repeating because it is the most common 125 litre mistake. Angels pair off and the pair evicts everyone else.
African cichlids: mbuna dilute aggression across a large group in a long tank. At 81 cm of length there is not enough territory to go round, and the dominant fish will kill the others one at a time.
Common plecos, oscars, and anything sold small that grows large: an oscar reaches 30-35 cm and needs 250 litres on its own. Check adult size before purchase, not after.
Filtration, cycling and maintenance
An external canister rated for 150-250 litres is the usual choice at this size and keeps the tank interior clear. Juwel tanks ship with an internal filter block that works adequately - many keepers upgrade the media rather than replacing the unit. Aim for moderate flow with a broken-up return so the angelfish are not swimming into a current all day.
Cycle first. At this volume you have real buffer, but a 125 litre also invites heavier stocking, which is exactly when an incomplete cycle bites. See how to cycle a fish tank.
Weekly 25-30% changes, with substrate vacuuming. A planted 125 litre with moderate stocking can stretch to fortnightly, but only if you are testing and the nitrates support it - see our water change frequency guide before deciding that.
Buy the largest water change container you will actually carry. The reason people skip water changes on a 125 litre is almost never forgetfulness - it is that 35 litres in a 10 litre bucket is seven trips. A 25 litre container or a hose-based changer turns a chore into ten minutes.
Track your parameters and get care reminders with App-aquatic.
Get the free appHow many fish can I keep in a 125 litre tank?
A comfortable community is two schools of 10-12 small fish, a bottom group of 6-8 corydoras, and one centrepiece pair - roughly 30-35 fish under 6 cm, or a pair of angelfish with a single school and corydoras. Adult size and waste output matter far more than the count, and the tall footprint means horizontal swimmers need more room than the volume suggests. Test the combination in our stocking calculator.
Can I keep angelfish in a 125 litre tank?
Yes - a pair, not a group. The 50 cm height of a typical 125 litre such as the Juwel Rio 125 is what makes angelfish viable at this size, where an 80 litre is too shallow. Keep one bonded pair, avoid neon and cardinal tetras as tank mates because adult angels will eat them, and avoid fin-nipping species like serpae tetras and tiger barbs entirely.
Is a 125 litre tank bigger than an 80 litre in a useful way?
It depends entirely on what you want to keep. The common Juwel Rio 125 is 81 cm long, which is the same length as many 80 litre tanks - the extra 45 litres is height. That height is genuinely useful for angelfish, gouramis and tall-bodied tetras, and close to useless for corydoras, loaches and fast horizontal swimmers. For a big shoal of small fast fish, a long 100 litre often beats a tall 125.
Can I keep discus in a 125 litre tank?
Not a group, and discus should not be kept singly. Five or six discus need 200 litres as a realistic minimum, water held at 28-30°C, and a water change routine well beyond a standard community tank. A 125 litre is better spent on an angelfish pair or a South American biotope, both of which give you similar visual impact without the maintenance load.
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