How Much Space Does a Sparkling Gourami Need?

A group of four to six sparkling gourami (Trichopsis pumila) needs an aquarium with a base of at least 45 × 30 cm, which is 18 × 12 inches, or roughly 40 litres and 10.7 US gallons at a standard 30 cm water depth. That base measurement is the figure Seriously Fish publishes for the species, and it is the number to plan around, because a fish with a maximum standard length of 4.0 cm (FishBase) does not run out of swimming room. It runs out of floor and surface to divide. A single pair fits a smaller tank, down to the 30 cm length Practical Fishkeeping gives for breeding pairs. Judged against the 1,350 cm² of floor that a 45 × 30 cm base produces, a standard 10-gallon (20 × 10 in) supplies 1,290 cm², about four per cent short, and a standard 5-gallon (16 × 8 in) supplies 826 cm², sixty-one per cent of it.

Where the 45 × 30 cm base comes from

Seriously Fish lists Trichopsis pumila at 40 mm maximum standard length with a recommended base of 45 × 30 cm, water at 22–28 °C, pH 5.0–7.5 and 1–12 dGH. FishBase records the maximum as 4.0 cm SL and places the species in ditches, small ponds and peat swamps across Thailand, Laos and Indonesia, in standing water under dense floating plants, sometimes low in oxygen. The IUCN assessment by Vidthayanon (2012) rates it Least Concern.

Practical Fishkeeping reaches the same figure from the other direction. Small groups want a tank "about 45cm (18") long or larger", while a breeding pair can go in one measuring 30 cm. Two sources landing independently on 45 cm as the group threshold is the strongest evidence on offer, because nobody has published a territory-size study for this species in aquaria. Treat 1,350 cm² as a sourced minimum.

Seven years of logging self-tan transfer against a towel chart taught me one transferable thing. The number printed on the bottle and the mark left on the fabric are two different measurements, and only one is evidence. Aquarium cartons have the same gap. A gallon rating is computed from outside dimensions, glass and trim included, and FishTankCalculator's reference chart says so outright: "published dimensions are nominal and rounded, so measure the inside of your tank before you trust a volume." Measure the base with a tape. That is the figure the fish live on.

Two 20-gallon tanks, two stocking limits

Standard glass aquariums come in fixed shapes, and two of them share a name.

| Tank (nominal) | Base | Floor and surface | Gross volume | Share of 1,350 cm² | |---|---|---|---|---| | 5 gallon | 16 × 8 in / 40.6 × 20.3 cm | 826 cm² | 21 L (5.5 gal) | 61% | | 10 gallon | 20 × 10 in / 50.8 × 25.4 cm | 1,290 cm² | 39 L (10.4 gal) | 96% | | 20 gallon high | 24 × 12 in / 61 × 30.5 cm | 1,858 cm² | 80 L (21.2 gal) | 138% | | 20 gallon long | 30 × 12 in / 76.2 × 30.5 cm | 2,323 cm² | 77 L (20.3 gal) | 172% | | 29 gallon | 30 × 12 in / 76.2 × 30.5 cm | 2,323 cm² | 112 L (29.6 gal) | 172% |

Volumes are gross internal figures from nominal dimensions, before substrate and freeboard, so a planted tank filled an inch below the rim holds less.

The 20 high and the 20 long carry the same name and nearly the same water. The long gives 2,323 cm² of floor against the high's 1,858, twenty-five per cent more territory at the same nominal capacity and usually the same price.

The 29 gallon makes the point harder. It sits on the identical 30 × 12 inch base as the 20 long, holds 46 per cent more water, and offers exactly the same floor and exactly the same water surface. Every extra litre is height, and height is the dimension a surface-oriented fish uses least.

That is where scaling fails. Water volume grows in three dimensions. The two resources sparkling gourami compete over, floor and surface, grow in two. Move from a 10 gallon to a 29 gallon and the water multiplies by 2.85 while the floor multiplies by 1.80. You pay for the first number and stock on the second.

How many sparkling gourami, and in what ratio

Four to six on a 45 × 30 cm base. Seriously Fish specifies "a pair or small group", and Practical Fishkeeping keeps pairs while warning that "two males will squabble". No inch-per-gallon rule survives contact with a 4 cm fish.

Ratio is where the hobby is least honest. One male to two females is repeated everywhere. I could not find a published sex-ratio study for T. pumila, and the behavioural data that does exist works against the assumption underneath that ratio. Ladich's 2020 experiment in the Journal of Experimental Biology staged same-sex contests using 17 males and 20 females, with sounds analysed from 12 males and 13 females. The sexes differed acoustically. Male croaks ran about 14 dB louder, males produced twice as many bursts per sound, and roughly 20 per cent of male bursts were single-pulsed against more than 80 per cent in females. They did not differ in contest duration, lateral display duration, number of sounds produced, or butting frequency. Females contest as hard as males do.

Stacking females to absorb male aggression is an assumption for this species, then, and not a measurement. My own preference is six unsexed juveniles bought together and left to settle, since reliable sexing means backlighting a female to see her ovaries beneath the swim bladder, which no shop tank permits.

Contests here stay cheap. Bischof's 1996 comparison of agonistic behaviour across three croaking gouramis and the paradise fish found attacks lowest in T. pumila, and heavily vocalising species show reduced visual and contact display next to near-mute relatives such as Macropodus. A dispute between males is mostly sound, flared fins and posture.

Same footprint as a betta, opposite stocking rule

Seriously Fish gives Betta splendens the same recommended base as the sparkling gourami: 45 × 30 cm, the identical 1,350 cm² of floor. On that identical floor the stocking conclusion inverts. That footprint holds one betta, since "only a single individual can be kept per tank in most cases" and males and females are housed separately outside breeding. The same footprint holds a group of T. pumila.

Size explains part of it. A betta reaches 70 mm SL against the sparkling gourami's 40 mm, so the larger fish takes the whole tank and the smaller one shares it. Tolerances differ as well: betta at 22–30 °C, pH 6.0–8.0 and 1–15 dGH, sparkling gourami at 22–28 °C, pH 5.0–7.5 and 1–12 dGH. The gourami wants cooler, softer, more acidic water.

The failure mode is specific. A nano keeper arriving from bettas has learned one fish, one territory, decoration chosen by eye. Sparkling gourami invert every clause: several fish, several territories, planting chosen to break sightlines. A layout that suits a betta holds a lone sparkling gourami you almost never see.

What extra volume actually buys

Volume scaling earns its keep in heat and chemistry.

Temperature first. Seriously Fish and Practical Fishkeeping both give 22–28 °C, and FishBase records the species at 25–28 °C in the wild. It has been cold since morning and it is cold now, one of those days when the season does not finish turning, and the tank room reads 17 °C. The heater in the 10 gallon cycles three or four times an hour to hold 25 °C. The 29 on the same wall cycles less and swings less, which is the thermal mass extra litres buy. A cold room also means cold air above the water, which a labyrinth fish breathes. Seriously Fish specifies a tight-fitting cover for breeding this species, and the same cover keeps the surface layer warm and humid the rest of the year.

Ammonia is where the arithmetic pays off. The EPA's 2013 freshwater criteria (EPA-822-R-13-001) set an acute limit of 17 mg/L total ammonia nitrogen and a chronic limit of 1.9 mg/L TAN, both normalised to pH 7.0 and 20 °C. Those are river criteria for whole ecosystems, far more generous than an aquarium should ever see.

What matters to a fish is un-ionised ammonia. SRAC Publication No. 463 (Durborow, Crosby and Brunson, 1997) puts short-term lethal exposure at roughly 0.6 mg/L un-ionised, with chronic gill and kidney damage and suppressed growth from 0.06 mg/L. Only a fraction of TAN takes that form, and the fraction tracks pH and temperature. SRAC's Table 1, calculated from Emerson et al. (1975), gives 0.60 per cent un-ionised at pH 7.0 and 26 °C, rising to 5.74 per cent at pH 8.0 at the same temperature.

Run the division. At pH 7.0 and 26 °C it takes 10 mg/L TAN to reach the 0.06 mg/L chronic threshold. At pH 8.0 it takes 1.05 mg/L. The soft acidic water this species prefers absorbs close to a tenfold error for every pH unit you drop, an underrated reason to keep them nearer pH 6.5 than 7.5.

Doubling the water halves the concentration a fixed waste load produces, so the 29 forgives a skipped water change the 5 gallon will not. White towels never keep a secret. Water is the reverse, since ammonia leaves no visible mark, and the test kit is the only chart you get.

The surface is the real territory

Tate and colleagues reviewed the labyrinth organ across roughly 137 anabantoid species in the Journal of Fish Biology in 2017 (volume 91, pages 723–749). All of them carry paired suprabranchial chambers housing the labyrinth apparatus, a bony, heavily vascularised air-breathing organ. FishBase calls T. pumila an obligate air-breather. The review is more careful, noting that gill reduction varies between genera and that both facultative and obligate air breathing occur in the group, so the claim that every labyrinth fish drowns without surface access is shaky. These fish still spend their working lives in the top few centimetres.

The same review notes that small Trichopsis and Macropodus take only small air bubbles into the pharyngeal cavity, which resonate at higher frequencies and sharpen hearing, with oxygen extraction primary and hearing secondary. The surface is a breathing station whose apparatus also services the acoustic contests described earlier.

Nests are built there too. Males construct bubble nests at the surface and, according to Seriously Fish, will not tolerate a female nearby until the nest is finished. Eggs hatch in 24 to 48 hours, and FishBase records nests holding roughly 100 to 170 eggs under male guard.

In a rectangular tank, surface area equals floor area. The 29 gallon offers a male the same 2,323 cm² of ceiling as the 20 long. Floating plants change that arithmetic. Salvinia, frogbit or a mat of water lettuce divides the ceiling into compartments a rival cannot see across, raising the number of defensible territories without altering one dimension of the glass. Covering half to two-thirds of the surface is the cheapest stocking upgrade this species has.

Which tankmates take surface space

Marbled hatchetfish (Carnegiella strigata) is the cleanest case. Seriously Fish requires gentle to moderate flow for them, insists on a tight cover because they jump when startled, and notes their food must stay floating long enough to be eaten. A shoal of them holds the top of the column full time and clears that floating food before it sinks. Sparkling gourami feed slowly and lose the race.

The same objection covers guppies, endlers and danios, all quick upper-column feeders, and male bettas, which claim the identical nesting surface and which Seriously Fish says are "best kept alone". Filter flow belongs on the list too. Seriously Fish specifies gentle water movement for T. pumila, and a return that ripples the surface flattens a bubble nest a male spent hours assembling.

Shrimp fail for the opposite reason. Seriously Fish notes the species "may prey on small freshwater shrimp", and Practical Fishkeeping calls them excellent shrimp hunters that take shrimplets and freshly moulted adults. An established Neocaridina colony out-breeds that predation in heavy planting. A starter group of twenty shrimp cannot.

Seriously Fish's compatible list runs to small schooling cyprinids and diminutive loaches, which sit below the contested layer and leave the ceiling alone.

Scaling a tank up without losing the fish

1. Measure the inside base with a tape and compare it against 1,350 cm². Ignore the carton's gallon rating.
2. Between two tanks of equal nominal volume, take the one with the larger base.
3. Cycle to zero ammonia, and hold pH nearer 6.5 than 7.5, where the un-ionised fraction stays around 0.6 per cent.
4. Set 24 to 26 °C and fit a close cover, so the air the fish breathe stays warm and humid.
5. Cover half to two-thirds of the surface with floating plants before the fish arrive, so territories exist on day one.
6. Add the group of four to six together rather than one fish at a time.

Frequently asked questions

Are sparkling gourami hard to keep?

No, with two conditions. They need stable warmth of 22–28 °C, soft acidic water at pH 5.0–7.5, and a cycled tank reading zero ammonia. The harder part is behavioural: they feed slowly at the surface and lose food to faster tankmates, so calm company works best.

How many sparkling gourami should be kept together?

Four to six on a 45 × 30 cm base, the minimum footprint Seriously Fish recommends. Practical Fishkeeping keeps single pairs in tanks from 30 cm long. Buy the group at once rather than adding fish later, since a resident male already holds the best nest sites.

How big of a tank does a sparkling gourami need?

A base of 45 × 30 cm (18 × 12 in), about 40 litres or 10.7 US gallons at 30 cm depth, per Seriously Fish. Judge tanks by that floor area of 1,350 cm² rather than the gallon label, because taller tanks on the same base add water without territory.

How many sparkling gourami can live in a 5-gallon tank?

One pair. A standard 5-gallon measures 16 × 8 inches, giving 826 cm² of floor, sixty-one per cent of the 1,350 cm² Seriously Fish recommends for a group. Its 16-inch length clears the 30 cm Practical Fishkeeping allows for a breeding pair, so two fit and a group does not.

Can sparkling gourami live alone?

A single fish will survive, though Seriously Fish describes the species as suited to a pair or small group. The croaking these gourami are named for is an agonistic signal produced during contests, documented by Ladich in staged pairs, so a solitary fish has little occasion to use it.

Which tankmates disrupt a sparkling gourami's surface territory?

Marbled hatchetfish, guppies, endlers and danios all hold the top of the water column and take floating food first. Male bettas claim the same nesting surface. Strong filter returns count too, since Seriously Fish specifies gentle flow and a rippled surface breaks a male's bubble nest.