Best Fish for a 40 Gallon Fish Tank
These species are well-suited to a 40-gallon aquarium in terms of adult size, bioload, and behaviour.
Discus (2โ3 juveniles)
A 40-gallon breeder is a practical starting point for a discus colony with excellent filtration.
Angelfish (4โ5)
The wide footprint of the 40 breeder gives angelfish territory they need as they mature.
Rainbow Fish (8โ10)
Boesemani or Australian rainbowfish shine in groups of 8+ in a 40-gallon.
Corydoras Catfish (8)
A larger cory group shows natural shoaling behaviour and cleans up bottom waste efficiently.
Cherry Barb (10โ12)
A large school of cherry barbs makes a striking red display. Males colour up beautifully in groups.
German Blue Ram (1 pair)
A breeding pair of rams in a 40 breeder is a classic setup; warm, soft water with corydoras companions.
40 Gallon Tank Dimensions & Weight
A standard 40-gallon aquarium measures about 36 ร 18 ร 17 inches (91 ร 46 ร 43 cm, L ร W ร H) and holds roughly 151 litres. Water weighs about 8.34 lb per US gallon (USGS water-density reference), so once you add glass, substrate and rock, expect a filled weight of around 382 lb. The 36 ร 18 inch footprint needs a stand rated for it; the wide base is also what makes this size so easy to aquascape and to net fish in.
Best for: dwarf cichlid pairs, planted aquascapes, or a large community that uses the wide floor. Not for: oscars or other large cichlids that outgrow it within a year.
Fish to Avoid at This Size
These species require more space, produce too much waste, or have specific needs that a 40-gallon cannot meet long-term.
Stocking & Water Change Guidelines
Stocking: 15โ25 small-to-medium fish, or 5โ8 medium fish (angelfish, discus) with supporting species.
Water changes: 20โ30% weekly, or 50% twice weekly for discus.
Cycle time: 4โ6 weeks. Larger tanks cycle more slowly but crash less dramatically.
Equipment & Setup Cost
| Equipment | Recommendation |
|---|---|
| Filter | Canister filter rated 80โ100 gal (Fluval 307, Oase BioMaster 350) โ the 40 breeder is often used for fish that need pristine water (discus, rams). |
| Heater | 150โ200 W adjustable. Two 100 W heaters for redundancy is common in discus setups. |
| Lighting | 40โ60 W LED. The wide footprint suits aquascaping layouts (Dutch, Iwagumi). |
| Estimated Setup Cost | $200โ$600 for a community setup; $400โ$1,200+ for a discus setup with a chiller. |
Frequently Asked Questions
What fish can live in a 40 gallon tank?
A 40-gallon tank โ particularly the popular 40 breeder (36ร18ร17 in) โ is large enough for angelfish (4โ5), discus (2โ3 juveniles), rainbow fish (8โ10), or a species tank with a pair of German blue rams and supporting corydoras. Its wide footprint is also popular for aquascaping, planted tanks, and breeding projects.
Is a 40 gallon tank good for discus?
40-gallon tank is a viable starting size for 2โ3 juvenile discus, but adult discus (8 inches) benefit from a 55-gallon minimum. Use a high-capacity canister filter, two heaters for redundancy, and perform 50% water changes twice weekly. Discus are sensitive to ammonia; a 40-gallon requires a mature biological filter before introducing them.
What is a 40 gallon breeder tank?
A 40 gallon breeder tank (36ร18ร17 in) is a wide, shallow aquarium designed originally for breeding fish. Its wide footprint gives more bottom and mid-level territory than a tall 40-gallon, making it a favourite for community tanks, planted aquascapes, and pairs of cichlids. It holds 40 US gallons and is one of the most versatile tank sizes in the hobby.
What size filter do I need for a 40 gallon tank?
For 40-gallon tank, use a canister or HOB filter rated for 80โ100 gallons. The Fluval 307 (rated up to 70 gal, but performs well beyond in practice) or the AquaClear 110 are popular choices. Over-filtering is rarely a problem and provides a safety buffer if you miss a water change.