How do I convert tank dimensions to gallons?

For rectangular tanks: multiply length × width × height in inches, then divide by 231. For liters, multiply cubic inches by 0.01639. Expect 10–15% less usable volume after substrate and decor.

How many fish can I keep?

The 1 inch per gallon rule is a starting guideline for small community fish. Large, messy, or territorial species like goldfish, oscars, and cichlids need significantly more space per individual.

Aquarium Size Calculator

Find the right tank size for your fish — with stocking density, filtration, heater wattage, and startup cost estimates.

Tank Setup

Fish Type

Decoration Level (reduces usable volume)

Filter Type

Quick Presets

Your Fish

Fish NameQtyAdult Size (in)

Recommended Tank Size

— gallons

Add fish to get your recommendation

Well Stocked

Fish: —" Type: 1.0× Deco: −15% Min: — gal Std: — gal

Stocking Density—%

Min. Gallons

—

Recommended

—

Stocking Density

—

Filter GPH

—

Heater Watts

—

Water Change/Wk

—

Add your fish to see the stocking chart.

Actions

Standard Aquarium Dimensions & Weight

The recommended row is highlighted gold based on your fish list.

Size (gal)	Dimensions (L×W×H)	Water Weight	Est. Full Weight	Typical Use
5 gal	16"×8"×10"	42 lbs	~55 lbs	Betta, shrimp nano
10 gal	20"×10"×12"	83 lbs	~111 lbs	Small community, nano
15 gal	24"×12"×12"	125 lbs	~170 lbs	Community starter
20 gal	24"×12"×16"	167 lbs	~225 lbs	Ideal beginner tank
29 gal	30"×12"×18"	242 lbs	~330 lbs	Community, fancy goldfish
30 gal	36"×12"×16"	250 lbs	~340 lbs	Community, planted
40 gal Breeder	36"×18"×16"	334 lbs	~458 lbs	Turtles, cichlids, breeding
55 gal	48"×13"×21"	459 lbs	~625 lbs	African cichlids, goldfish
65 gal	36"×18"×24"	543 lbs	~772 lbs	Large community, saltwater
75 gal	48"×18"×21"	626 lbs	~850 lbs	Oscar, large cichlids
90 gal	48"×18"×24"	751 lbs	~1,050 lbs	Reef, large freshwater
125 gal	72"×18"×22"	1,043 lbs	~1,400 lbs	Show tank, large reef
150 gal	72"×18"×28"	1,250 lbs	~1,700 lbs	Specimen fish, large reef
180 gal	72"×24"×25"	1,502 lbs	~2,100 lbs	Expert setups, large schools
210 gal	72"×24"×29"	1,752 lbs	~2,400 lbs	Public-style display

Tank Placement Guidelines

Residential floors typically support 40–50 lbs/sq ft. Large tanks (>90 gal) may need floor reinforcement.
Place tanks away from direct sunlight to prevent algae blooms and temperature swings.
Ensure a perfectly level surface — even ¼" off level can stress glass seams over time.
Allow 12"+ of clearance behind the tank for equipment access and maintenance.
Keep electrical outlets nearby but use a drip loop on all cords to protect from splashes.
Avoid exterior walls in cold climates — temperature fluctuations stress fish.

Custom Tank Volume Calculator

Know your tank's exact dimensions? Calculate the true volume and maximum fish load.

Length (in)

Width (in)

Height (in)

Water Parameters by Tank Type

Matching water chemistry to your fish type is as important as tank size.

Tank Type	Temp (°F)	pH Range	Hardness	Salinity / Notes
Tropical Freshwater	76–80	6.8–7.5	2–15 dGH	Dechlorinated tap water fine
Goldfish (coldwater)	60–72	7.0–7.4	10–25 dGH	No heater needed
African Cichlids	76–82	7.8–8.5	10–20 dGH	Hard, alkaline water; crushed coral substrate
Betta	78–82	6.5–7.5	1–5 dGH	Soft, slightly acidic; gentle flow
Marine / Reef	76–78	8.1–8.4	8–12 dKH	Salinity: 1.024–1.026 SG; RO/DI water required
Planted (low-tech)	72–78	6.5–7.2	3–8 dGH	CO₂ optional; high-output lighting

The Nitrogen Cycle — Never Skip It

Adding fish to an uncycled tank causes "New Tank Syndrome" — ammonia spikes kill fish within days. Allow 4–6 weeks before stocking.

Phase	Timeline	What Happens	Action
Ammonia Build	Days 0–14	Ammonia rises from fish waste & food	Add ammonia source; test daily
Nitrite Spike	Days 14–28	Nitrosomonas bacteria: NH₃ → NO₂⁻	Ammonia falls, nitrite rises
Nitrate Rise	Days 21–42	Nitrobacter bacteria: NO₂⁻ → NO₃⁻	Both fall; nitrate climbs
Cycled!	4–6 weeks	NH₃ = 0, NO₂⁻ = 0, NO₃⁻ < 40 ppm	Safe to add fish gradually

Common Fish — Size, Difficulty & Min Tank

Fish	Adult Size	Difficulty	Temp (°F)	Min Tank	Notes
Neon Tetra	1.5"	Beginner	72–80	10 gal	School of 6+; peaceful
Guppy	2"	Beginner	72–82	10 gal	Livebearers; breed rapidly
Zebra Danio	2"	Beginner	65–77	10 gal	Hardy; great for cycling
Platy	2.5"	Beginner	70–82	10 gal	Colorful; livebearers
Corydoras	2.5"	Beginner	72–80	20 gal	Bottom dwellers; school of 4+
Molly	3"	Beginner	72–82	20 gal	Livebearers; tolerate brackish
Betta	3"	Beginner	78–82	5 gal	Solitary; no other bettas
Harlequin Rasbora	2"	Beginner	72–80	10 gal	School of 8+; planted tanks
Fancy Goldfish	8"	Intermediate	60–72	20 gal	Coldwater; no heater
Common Goldfish	12"	Intermediate	60–72	55 gal	Coldwater; ponds preferred
Angelfish	6"	Intermediate	76–82	29 gal	Tall tank needed; semi-aggressive
German Blue Ram	3"	Intermediate	78–85	20 gal	Sensitive to water quality
Kuhli Loach	4"	Intermediate	74–82	20 gal	Nocturnal; needs hiding spots
Discus	6"	Advanced	82–88	55 gal	Very sensitive; pristine water only
Oscar	14"	Advanced	74–81	75 gal	Messy; needs 10× GPH filter
Clownfish (Marine)	4"	Advanced	76–82	20 gal	Reef-safe; needs anemone
Blue Tang (Marine)	12"	Expert	72–82	100 gal	Needs large open swimming space

Frequently Asked Questions

What is the inch-per-gallon rule? +

The inch-per-gallon rule states that you need 1 gallon of water for every 1 inch of adult fish length. A 2" neon tetra needs 2 gallons; five of them need 10 gallons. This is a starting guideline — large fish (over 3") need more space because body mass and waste production scale faster than length. This calculator applies species-specific corrections automatically.

Why do goldfish need so much space? +

Common goldfish grow up to 12" and are exceptionally heavy waste producers. The old "bowl" idea is a harmful myth — goldfish kept in bowls suffer stunted growth and die young. A single common goldfish needs at least 20 gallons, with 10 additional gallons per extra fish. Fancy goldfish are smaller but still need 20+ gallons each.

How often should I change the water? +

For a properly cycled, appropriately stocked tank: 25–30% weekly. Overstocked tanks need larger, more frequent changes. Always use a dechlorinator when adding tap water and match the temperature to avoid temperature shock. Test water parameters (ammonia, nitrite, nitrate, pH) weekly for new tanks, monthly once established.

What size filter do I need? +

Your filter should turn over the full tank volume 4–5× per hour (GPH). For a 55-gallon tank, that's 220–275 GPH minimum. For marine tanks, goldfish, and cichlids, target 6–10× turnover. It's better to over-filter — there's no such thing as water that's too clean. This calculator shows the minimum recommended GPH for your filter type.

How much does a fish tank cost to set up? +

A 20-gallon freshwater starter tank typically costs $150–$300 all-in (tank, filter, heater, substrate, lighting, test kit, décor). A 55-gallon community tank runs $350–$600. Marine/reef tanks cost 3–4× more due to live rock, protein skimmer, and specialized lighting. Use the "Startup Cost" chart tab inside the calculator for a customized estimate.

How to Use This Calculator

Select Your Fish Type

Choose Tropical, Goldfish, Marine, Cichlid, or Betta. Each applies the correct stocking multiplier — marine fish need 2× the space of equivalent freshwater species.

Add Your Fish

Enter each species, how many you want, and their adult size in inches. Use Quick Presets to populate common setups instantly. The calculator updates in real time.

Review All Results

Get your recommended tank size, stocking density, filter GPH, heater wattage, weekly water change schedule, and a full startup cost estimate — all in one place.

Formula & Methodology

Stocking (Small Fish ≤ 3") gallons = Σ(qty × size) × type_mult 1 gallon per 1 inch of adult fish length, then multiplied by type factor (marine = 2×, cichlid = 1.5×).

Stocking (Large Fish > 3") gallons = qty × (size² / 3) × type_mult Large fish body mass scales with the square of length, so space requirements grow disproportionately.

Decoration Factor adjusted = base ÷ (1 − deco_reduce) Moderate décor reduces usable volume by 15%; heavy décor by 25%. Adds back the displaced volume.

Filter Turnover GPH = tank_gal × turnovers_per_hour Power: 4×, Canister: 5×, Sump: 6×, Undergravel: 3×. Messy fish or marine tanks should target the high end.

Key Terms

Bioload

Total waste produced by fish and food. Higher bioload = stronger filtration needed. Goldfish and oscars have very high bioload.

Nitrogen Cycle

Beneficial bacteria convert toxic ammonia → nitrite → nitrate. Takes 4–8 weeks to establish in a new tank.

Stocking Density

The ratio of your fish's space requirements to total tank volume. Under 70% is ideal; over 80% is heavily stocked.

Turnover Rate

How many times per hour your filter processes the full tank volume. Minimum 4× for tropical; 6–10× for goldfish/marine.

Hardscape

Non-living décor (rocks, driftwood, ornaments) that displaces water volume and reduces effective stocking capacity.

Specific Gravity (SG)

The density of saltwater relative to fresh. Marine fish need 1.024–1.026 SG — measured with a hydrometer or refractometer.

Real-World Examples

EXAMPLE 1

20-Gallon Beginner Community

5 × Neon Tetra (1.5") + 2 × Guppy (2") + 1 × Bristlenose Pleco (4")

Fish load: 7.5 + 4 + 5.33 = 16.8 gal × 1.0× type × ÷0.85 deco = 19.8 gal → min 24 gal → 29 gal std

Result: 29-gallon tank, power filter at ≥120 GPH, 100W heater, ~25% weekly water change.

EXAMPLE 2

55-Gallon African Cichlid Tank

3 × Yellow Lab (4") + 3 × Demasoni (3") | Fish type: Cichlid (1.5×)

Load: (3×5.33 + 3×3) × 1.5 = 37.5 gal × deco → min 53 gal → 55 gal std

Result: 55-gallon tank, canister filter at ≥275 GPH, heavy rockwork, 30% weekly water changes.

EXAMPLE 3

Marine Reef Starter

2 × Clownfish (3") + 1 × Royal Gramma (3") | Fish type: Marine (2.0×)

Load: (2+1) × 3 × 2.0 = 18 gal × deco → min 25 gal → 29 gal std

Result: 29-gallon reef, sump filter at ≥175 GPH, protein skimmer, live rock, est. $850+ startup.

Aquarium Setup: Volume, Stocking & Filtration

Setting up a healthy aquarium requires understanding the relationship between tank volume, fish stocking density, and filtration capacity. The most common cause of fish death in home aquariums is poor water quality from a disrupted or incomplete nitrogen cycle — getting the chemistry right from day one prevents this entirely.

The Nitrogen Cycle: Your Tank's Biological Foundation

Fish waste and uneaten food produce ammonia, which is acutely toxic to fish. Beneficial bacteria (Nitrosomonas) convert ammonia to nitrite; a second species (Nitrobacter) converts nitrite to much safer nitrate. Nitrate is removed through water changes. This cycle takes 4–8 weeks to establish in a new tank. Never add fish to an uncycled tank — the ammonia spike will kill them within days.

Why Stated Tank Size Differs from Actual Volume

Your tank's stated size (20 gallons, 55 gallons) represents total glass capacity, not usable water volume. Substrate, rocks, decorations, and driftwood displace water. A heavily aquascaped 55-gallon may hold only 45 gallons of actual water. This calculator accounts for decoration displacement automatically — use the "Decoration Level" chip to select the closest match to your planned setup.

Filtration: Size Up, Not Down

Filter manufacturers rate products in gallons per hour (GPH). A filter rated for your exact tank size is almost always insufficient — always oversize by 50–100%. For messy fish like goldfish or heavily stocked cichlid tanks, target 8–10× hourly turnover. Clean filter media monthly using saved tank water (tap water kills beneficial bacteria). Replace chemical media every 4–6 weeks but leave biological media alone.

Marine vs. Freshwater: The Cost Difference

A marine reef aquarium costs 3–4× more than an equivalent freshwater setup. Live rock ($6–10/lb, 1–1.5 lbs/gallon), a protein skimmer ($80–400), RO/DI water system ($150+), and reef lighting ($200–600+) are all additional requirements. Marine tanks are also less forgiving of mistakes — water chemistry swings that would stress freshwater fish can kill reef inhabitants within hours.