Fish Tank Size & Stocking Calculator: Smart, Science-Based & Beginner-Friendly
Planning your aquarium is half the hobby. Whether you’re setting up a heated 20-liter betta tank or mapping a lush, planted community, the question is always the same: “How many fish can I keep—safely?”
This guide explains how the Fish Tank Size & Stocking Calculator works, why it beats old one-line rules, and how to use it to keep your fish healthy and your water crystal clear.
Why This Calculator Exists (and what it fixes)
Overstocking is the #1 beginner mistake. Too many fish = too much waste, leading to ammonia and nitrite spikes, stress, disease, and algae. Traditional shortcuts like “1 inch per gallon” (or 1 cm per liter) ignore important realities:
- A 5 cm neon tetra and a 5 cm fancy goldfish produce very different bioloads.
- Body shape and activity matter (slender schoolers vs. deep-bodied or messy species).
- Tank shape (surface area), plants, and filtration change how much life a tank can support.
The calculator builds a plan around actual volume, species behavior, waste output, and your maintenance habits—so you stock smart from day one. Even welfare orgs that publish length-based guides stress they’re only rough guides; real capacity depends on filtration, husbandry, and fish species.
What the Calculator Considers
Tank Volume & Shape
Rectangles and cylinders calculate differently; then we reduce to usable volume (subtracting 10–25% for substrate, wood/rock, and internal gear).
Species & Size (Beginner Mode vs. Expert Mode)
- Beginner Mode: choose by size class for quick, safe estimates.
- Expert Mode: pick from 200+ species with bioload factors and schooling requirements baked in.
Bioload & Filtration
The tool estimates total bioload (a proxy for waste), compares it to usable volume, and maps it to a Tank Load Tier (Very Light → Overstocked). You’ll see water-change advice and filter turnover tips matched to the tier.
Behavior & Compatibility
Schooling needs, territorial space, temperature range, and temperament are flagged so you avoid stressful mixes.
How the Math Works (Under the Hood)
Volume
- Rectangle: L × W × H
- Cylinder: π × r² × H
- Usable volume = display volume – décor displacement (your chosen %).
- Conversions: 1 US gal = 3.785 L, 1 Imp gal = 4.546 L.
Beginner-Mode Stocking (safe starting heuristic)
Totals length for small, slender fish only and applies a conservative cm/L (or in/gal) guide.
Important: this heuristic is not valid for deep-bodied or high-waste species (e.g., goldfish, many cichlids). That mirrors welfare guidance: length rules = rough guide only, not a law. The UI warns when it doesn’t apply.
Expert-Mode Stocking
- Species Bioload = quantity × average adult size × bioload factor.
- Sum across species → Total Bioload Index; compare to usable volume for your Load %.
Tank Load Tiers (example mapping)
- Very Light: ≤ 40% — forgiving; beginner-friendly
- Light: 41–60% — stable with routine maintenance
- Moderate: 61–80% — watch feeding; plants/filtration help
- Heavy: 81–100% — experienced keepers; strict maintenance
- Overstocked: > 100% — not recommended; reduce stock or upgrade
Why this beats “1 cm per liter”: Metabolic waste scales with mass, not length; mass rises faster than length. Bioload factors correct for that. (Even experienced hobby references critique the inch-per-gallon rule for this reason.)
The Science You’re Managing (one minute)
Nitrogen Cycle
Fish and food create ammonia (NH₃/NH₄⁺). Beneficial bacteria convert it to nitrite (NO₂⁻), then to nitrate (NO₃⁻). In aquarium biofilters, Nitrosomonas (AOB) and Nitrospira (NOB) are the main players; recent studies also find comammox Nitrospira (capable of oxidizing ammonia all the way to nitrate) commonly present in freshwater aquaria.
Filtration & Turnover
Manufacturer flow ratings are optimistic. A practical baseline is at least ~4× turnover per hour; aim for ~4–6× in planted/community tanks and ~6–10× for messy species (goldfish, many cichlids) or high-flow hillstream setups.
- LiveAquaria
- Practical Fishkeeping
Plants Help
Fast-growing aquatic plants preferentially take up ammonium (NH₄⁺) and can materially reduce nitrogenous waste when actively growing. This stabilizes water and provides cover—but it’s not a license to overstock.
- ScienceDirect
- Diana Walstad's Books and Articles
Surface Area & Gas Exchange
Long, shallow tanks have more air–water interface than tall, narrow ones, improving oxygenation—especially with a gentle ripple or trickling/stripping action.
Species Notes that Change Everything
- Goldfish (fancy/comet): High bioload, constant grazing, big O₂ demand. Think volume + strong filtration first. Welfare guidance sets an absolute minimum around 50 L for a single goldfish (with appropriate dimensions). In practice, many keepers find ~75–95 L or more gives a more stable margin for a single fancy goldfish—then plan up from there as it grows. RSPCA Knowledgebase
- Bettas (Betta splendens): Labyrinth breathers still need heated, filtered water. A ~20 L (≈ 5 US gal) practical minimum has solid backing in welfare and veterinary guidance; use a gentle-flow filter and a heater. RSPCA Knowledgebase; Veterinary Medicine at Illinois
- Schooling Tetras/Rasboras: Keep 6–10+ of each species; small bodies, moderate bioload; thrive in planted setups.
- Livebearers (guppies, platies): Fast breeders—plan for population growth or stick to single-sex groups.
- Dwarf Cichlids (Apistogramma, Rams): Territory matters. Provide footprint, caves, and line-of-sight breaks.
- Rift Cichlids/Mbuna: Often stocked more densely to spread aggression, but this demands heavy filtration and strict maintenance—not a beginner path unless following a proven recipe.
- Catfish & Plecos: Many species outgrow small tanks and are heavy waste producers. Research adult size; bristlenose plecos are often the sensible choice.
Water Changes & Maintenance by Load Tier (starting guidance)
- Very Light: 20–25% every 2 weeks; gentle feeding; prune plants.
- Light: 25–30% weekly; light gravel-vac.
- Moderate: 30–40% weekly; clean prefilter sponges; monitor nitrate.
- Heavy: 40–60% weekly (or split twice weekly); rinse media in tank water, not tap (chlorine/chloramine harm biofilter bacteria). RSPCA Knowledgebase
- Overstocked: Reduce stock, upgrade filtration, or increase change frequency—now.
Example Walk-Throughs
- 1) 75 L planted community (rectangle), usable 65 L
Stock: 10 ember tetras (2 cm), 8 chili rasboras (2 cm), 6 pygmy corys (3 cm), 1 nerite.
Result: Light → Moderate load. Weekly 25–30% changes, ~4–6× turnover, floating plants for extra polish. - 2) 90 L “goldfish starter,” usable 75 L
Stock: 1 juvenile fancy goldfish.
Result: Moderate now; will trend Heavy as it grows. Plan a larger tank or strong external filter; 40–50% weekly changes. - 3) 20 L heated betta tank, usable 17 L
Stock: 1 male betta, 5 shrimp, 3–4 snails.
Result: Very Light → Light with plants. Weekly 25–30% changes; baffle filter outflow.
Compatibility & Schooling—Don’t Skip This
- Schooling requirement: Many “nano” fish need 6–10+ of their own species to feel safe and show natural behavior.
- Temperament: Fin-nippers (some barbs, serpae tetras) can harass long-finned fish.
- Temperature & water: Keep species within a shared range; mixing cool-water danios with warm-water bettas stresses both.
- Territory: Provide hardscape and plants to break sightlines and reduce aggression.
Quick Glossary
- Bioload: Waste burden (ammonia, dissolved organics) a fish produces.
- Usable Volume: Free water after subtracting décor/substrate displacement.
- Tank Load: Calculated strain on filtration/maintenance capacity.
- Turnover: Filter output per hour relative to tank volume (×/hr).
- Cycle/Cycling: Establishing beneficial bacteria before fully stocking.
- Quarantine: Isolating new fish 2–4 weeks prevents disease outbreaks.
FAQs
Is the 1 inch per gallon (1 cm per liter) rule true?
Treat it as a rough starter for small, slender fish only. Real capacity depends on mass/bioload, filtration, oxygenation, and husbandry. The calculator’s bioload factors are more realistic.
Do live plants let me keep more fish?
They stabilize water and absorb nitrogen (especially ammonium) when actively growing—but they don’t excuse overstocking.
- ScienceDirect
- Diana Walstad's Books and Articles
How much filter flow do I need?
Use at least ~4× turnover as a baseline; ~4–6× for planted/community tanks; ~6–10× for messy species or hillstream/river setups that prefer brisk current.
- LiveAquaria
- Practical Fishkeeping
How fast can I add fish?
After cycling, add in small batches, testing ammonia/nitrite. Spikes mean slow down.
What’s a good minimum for a betta?
~20 L (≈ 5 US gal) with a heater and gentle filtration.
- RSPCA Knowledgebase
- Veterinary Medicine at Illinois
How should I clean my filter?
Rinse media in tank water, not under the tap, to protect biofilter bacteria from chlorine/chloramine.
Safety & Ethics Disclaimer
This tool offers evidence-based guidance, but real tanks vary with feeding, filtration, aquascape, and husbandry. Always research individual species, buy responsibly, quarantine new arrivals, and err on the side of under-stocking and over-filtering. Healthy fish are active, eating well, and living in clear, stable water.
Bottom line
Plan with the calculator, stock slowly, test your water, and enjoy a thriving tank. 🐟💧
Sources & useful reads
- Nitrogen cycle in home aquaria: Dominance of Nitrosomonas/Nitrospira and presence of comammox Nitrospira in freshwater aquarium biofilters. — ASM Journals; PMC
- Length-based rules are only rough guides: RSPCA guidance; critique of inch-per-gallon heuristics. — RSPCA; aquariumscience.org
- Filter turnover bands: Baseline ~4× and when to go higher; high-flow hillstream guidance. — LiveAquaria; Practical Fishkeeping
- Plants & ammonium uptake: Research review + Walstad article. — ScienceDirect; Diana Walstad's Books and Articles
- Surface area & gas exchange: Aeration/stripping and maximizing interface. — Eurofish; web.utk.edu
- Betta care minimums: RSPCA Knowledgebase; University of Illinois Vet Med. — RSPCA Knowledgebase; Veterinary Medicine at Illinois
- Goldfish minimums: RSPCA Knowledgebase (minimum ~50 L; bigger is better). — RSPCA Knowledgebase