Calculating Stocking Density
Ditch the "inch per gallon" rule! Learn scientific methods to calculate safe stocking levels for your aquarium.
besatz · 6 Min. Lesezeit · stocking density, bioload, planning, overstocking
Calculating Stocking Density
Overstocking is one of the most common aquarium problems. Let's break down how to calculate appropriate fish loads scientifically.
Why "Inch Per Gallon" Fails
The old rule: 1 inch of fish per gallon (or 2.5cm per 4 liters)
Why it's dangerously wrong:
- A 30cm Oscar ≠ 30 Neon Tetras (1cm each)
- Ignores bioload (waste production)
- Ignores swimming space needs
- Ignores territorial behavior
- Uses juvenile size, not adult size
Real example: A 12-inch Oscar needs 300 liters minimum, not 48 liters!
Better Calculation Methods
Method 1: Surface Area Rule
More accurate for most fish:
1cm of fish per 12 square cm of surface area
Surface area = Length × Width (top of tank)
Example: 100×40cm tank
- Surface area: 4,000 cm²
- Fish allowance: 4,000 ÷ 12 = ~333cm total fish length
- Could stock: 30 fish at 10cm adult size OR 15 fish at 20cm adult size
Still imperfect, but better than volume alone.
Method 2: Bioload Calculation
Different fish produce different waste amounts:
Light Bioload (1× multiplier):
- Small schooling fish (Tetras, Rasboras, Danios)
- Herbivores and omnivores
- Corydoras, Otocinclus
Medium Bioload (2× multiplier):
- Livebearers (constant breeding = waste)
- Gouramis
- Barbs
Heavy Bioload (3-4× multiplier):
- Large carnivores (Oscars, larger Cichlids)
- Goldfish (messy eaters, high waste)
- Plecos (massive poop machines)
Example:
- 10 Neon Tetras (light) = 10 units
- 2 Angelfish (medium) = 4 units
- 1 Oscar (heavy) = 12 units
Adjust stocking to total bioload units, not just numbers.
Method 3: AqAdvisor.com
The easiest and most comprehensive method:
- Enter tank dimensions
- Add your desired fish (database knows adult sizes)
- Enter your filter brand/model
- Selects maintenance schedule
AqAdvisor calculates:
- Stocking level percentage
- Filtration capacity vs. bioload
- Warnings for aggression/compatibility
- Recommendations for improvement
Aim for 80-85% stocking on AqAdvisor — leaves room for growth and error.
Factors That Increase Maximum Stocking
Heavy Filtration
Oversized filters (rated 2-3× tank volume) can handle higher bioloads.
Examples:
- Canister filter rated 600 L/h on a 200L tank
- Two filters running simultaneously
- Sponge filters + HOB filters combined
Frequent Maintenance
More water changes = higher stocking possible
- 50% water change weekly → Can stock moderately
- 25% water change twice weekly → Can stock higher
Live Plants
Heavily planted tanks consume nitrates and provide oxygen, allowing slightly higher stocking.
Best nutrient-hungry plants:
- Fast growers: Hornwort, Water Sprite, Hygrophila
- Floaters: Frogbit, Red Root Floaters
Plants ≠ No water changes! Still need regular maintenance.
Factors That Decrease Maximum Stocking
Aggressive Fish
Territorial fish need space!
- African Cichlids: Often need 1 fish per 15-20 liters
- Bettas: Territorial, keep solo or with careful planning
Poor Filtration
Undersized filter = lower stocking capacity
If your filter is rated for 100L but your tank is 150L, stock conservatively!
Irregular Maintenance
If you only do monthly water changes, stock very lightly or upgrade your routine!
Practical Stocking Examples
60L (15 Gal) Tank — Light Stocking
- 8 Chili Rasboras
- 6 Ember Tetras
- 6 Pygmy Corydoras
- 5 Cherry Shrimp
AqAdvisor: ~70% stocked
200L (50 Gal) Tank — Moderate Stocking
- 15 Neon Tetras
- 12 Harlequin Rasboras
- 8 Corydoras Sterbai
- 2 German Blue Rams
- 1 Bristlenose Pleco
- 10 Amano Shrimp
AqAdvisor: ~85% stocked
400L (100 Gal) Tank — Heavy (But Safe) Stocking
- 30 Cardinal Tetras
- 20 Rummy-nose Tetras
- 15 Corydoras (mixed species)
- 6 Angelfish
- 3 Bristlenose Plecos
- 20 Cherry Shrimp
AqAdvisor: ~90% stocked
Requires: Canister filter, 40% weekly water changes, heavy planting
Warning Signs of Overstocking
- Ammonia/nitrite above 0 despite cycling
- Nitrate climbs above 40ppm between water changes
- Fish gasping at surface
- Aggression and fin-nipping
- Stunted growth
- Frequent disease outbreaks
Conclusion
Stock conservatively, especially as a beginner. You can always add more fish, but removing them is stressful.
Best Practice:
- Use AqAdvisor for initial planning
- Stock to 70-80% maximum
- Monitor water parameters weekly
- Adjust stocking if nitrate trends upward
Fewer, healthier fish beats a crowded, stressed tank every time!
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