Fish Stocking & Bioload Capacity Calculator

Estimate a safe stocking range from your tank’s net volume, filtration, maintenance routine, and fish type. Results include bioload capacity and a suggested fish-count range based on adult size and waste level.

Tank & System

Units Net Water Volume

gal

If unsure, use your tank’s rated gallons and subtract ~10–15% for substrate/decor. Fish System / Species Group Heavily Planted? Dense, fast-growing plants can offset light bioload (+10%).

Filtration & Maintenance

Filter Type Rated Flow

gph

Factory GPH/LPH. The tool auto-adjusts for media & head loss. Weekly Water Change

Higher routine changes increase safe capacity (to a point). Extra Aeration / Surface Agitation Air stone, spray bar, ripples can slightly raise capacity.

Fish Size & Waste (for Count Estimate)

Typical Adult Length

Use mature size (e.g., neon tetra 1.2", platy 2.5", oscar 12"). Waste / Activity Level

Safe Stocking Capacity

Enter tank, filtration, and species group to see a conservative, standard, and experienced stocking range.

Effective Turnover

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Adjusted for filter type & media loss

Bioload Capacity (BP)

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Abstract units proportional to sustainable waste

Suggested Fish Count

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Conservative • Standard • Max (experienced)

Recommended Flow

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For your selected group

How the Fish Stocking Calculator Works

Instead of the outdated “inch-per-gallon” rule, this tool estimates an abstract Bioload Capacity (BP) from your net water volume, filtration turnover, species group, and maintenance routine. BP is then converted into a suggested fish count using your typical adult length and waste level.

Model Overview

Base capacity: proportional to net gallons (or liters).
Species factor: goldfish & large cichlids reduce capacity; shrimp/nano/planted slightly increase it.
Filtration factor: compares your adjusted turnover (GPH ÷ gallons) to the group’s recommended turnover.
Maintenance factor: routine water-change percent nudges safe capacity up/down (diminishing returns).
Fish cost: each fish “uses” BP ≈ length^1.5 × waste multiplier (low/med/high).

Stocking Guidelines & Examples

Freshwater Community (standard maintenance)

For a 40 gal with ~5× turnover and 30% weekly changes, 2" medium-waste fish (platies, barbs) typically fall in the 12–18 fish range depending on planting, aquascape, and fish temperament.

Goldfish

Goldfish are messy and active. Aim for higher turnover (8×+) and frequent water changes. A 40 gal with adequate filtration suits 1–2 fancy goldfish or 1 single-tail juvenile with plans to upgrade.

Reef Systems

Marine tanks demand high gas exchange and strong export (skimmer, refugium). Even with 10× turnover, reef stocking should stay conservative, favoring smaller, compatible species.

FAQs — Bioload, Turnover & “Inch-Per-Gallon”

Is “inch-per-gallon” accurate?

Not really. Waste scales faster than length, and filtration/maintenance matter. This tool models both.

What turnover should I target?

Community: ~5× • Planted: 4–5× gentle flow • Goldfish: 8× • African cichlids: 7× • SA/CA cichlids: 6–7× • Marine FOWLR: 8–10× • Reef: 10× (plus skimming).

How do plants affect stocking?

Dense, fast-growing plants can absorb some nitrogenous waste; we add a small allowance (+10%).

Why is my result a range?

Because temperament, feeding, and aquascape vary. We show conservative, standard, and an experienced keeper’s upper bound.