🐠 Aquarium Stocking Calculator
Calculate Safe Fish Capacity Using Surface Area + Bioload Method
Aquarist • 18 years experience • 15 display tanks • r/PlantedTank moderator
Tank Dimensions
Fish to Stock
🎯 Expert Tips from Marcus Lee
- Surface area > volume for oxygenation: I see so many beginners buy tall tanks because they "look cool." A 20-gallon long (30x12x12") holds WAY more fish than a 20-gallon high (24x12x16") despite same volume. Why? Surface area = oxygen exchange. Tall tanks are fish coffins unless you run serious aeration.
- The "swimming room" rule nobody mentions: Active swimmers (danios, barbs, rainbowfish) need 6x their body length in horizontal swim space. A 6-inch rainbow in a 36-inch tank? That's torture. Meanwhile, bottom-dwellers (corys, loaches) need FLOOR space more than height. Match tank shape to fish behavior, not just size.
- Filtration is your safety margin: I run 10x turnover on all my tanks because life happens—you forget a water change, overfeed once, a fish dies and decomposes. Overfiltration is impossible; underfiltration is deadly. My 40-gallon breeder runs a FX4 (700 GPH) + sponge filter. Overkill? Never had an ammonia spike in 7 years.
- Quarantine isn't optional: Every single fish gets 4 weeks in my 10-gallon QT. I've dodged ich, velvet, and columnaris outbreaks because of this. New fish from stores are disease vectors. Skipping QT to "save time" means losing your entire display tank. Trust me, I learned the $800 lesson once. Never again.
⚠️ Common Mistakes to Avoid
- The "fully grown" lie pet stores tell: Employee says "common plecos stay 6 inches." LIAR. They grow to 24 inches and produce more waste than three goldfish. A kid at my LFS sold someone a clown loach for a 20-gallon—those reach 12 inches and need 75+ gallons. Google "[species name] + SeriouslyFish" for REAL adult sizes, not sales pitches.
- Trusting AqAdvisor blindly: AqAdvisor is a great tool, but it doesn't account for aggression or compatibility. I once followed it for a mbuna cichlid setup—95% stocking level looked safe. Reality? Aggression-driven deaths within 2 months because I didn't overstock ENOUGH (mbuna need crowding to diffuse aggression). Use calculators as guides, not gospel.
- The "one more fish won't hurt" spiral: You start with 10 neons. Add 5 corys. Then a centerpiece gourami. Then "just 3 more" rasboras because the school looks thin. Suddenly you're overstocked, nitrates hit 80ppm, and you're doing 50% water changes weekly to keep fish alive. I've been there. Stick to your stocking plan like it's a diet—cheat days kill fish.
- Ignoring behavior, only counting inches: I see people cramming 6 angels in a 55-gallon because "it's only 36 inches of fish!" Angels are TERRITORIAL as hell. You need 10 gallons PER angel minimum, or they'll murder each other. Same with bettas, cichlids, and even "peaceful" gouramis. Aggression bioload is real and calculators can't measure it.
Understanding Aquarium Stocking: Science Over Rules of Thumb
Why "1 Inch Per Gallon" is Dangerously Wrong
The classic "1 inch of fish per gallon" rule has killed more fish than ich. Here's why it's garbage:
Problem 1: Shape matters
A 2-inch-long, pencil-thin neon tetra ≠ a 2-inch-tall, boxy goldfish. The goldfish has 5x the body
mass and produces 5x the waste.
Problem 2: Activity level
A lazy pleco and a hyperactive danio both measure 3 inches, but the danio needs 10x more swimming
room.
Problem 3: Adult size confusion
Stores sell 2-inch "baby" oscars. They grow to 12-14 inches. Your 30-gallon tank can hold "fifteen
2-inch fish" by the rule, but ONE adult oscar needs 75+ gallons.
The Better Method: Surface Area + Bioload
Surface Area Formula:
Stocking capacity = (Length × Width) ÷ 12 sq inches per inch of slim-bodied fish
Example: A 30x12" tank = 360 sq inches ÷ 12 = 30 inches of slim fish (like tetras, rasboras).
But then adjust for bioload:
- Low bioload (tetras, rasboras, guppies): Use full capacity
- Medium bioload (mollies, platys, bettas): Reduce by 30%
- High bioload (goldfish, cichlids, plecos): Reduce by 50-70%
So that same 30-gallon long can hold:
- 30 inches of neon tetras (20 fish) ✅
- OR 21 inches of mollies (7 fish) ✅
- OR 12 inches of goldfish (2 fish) ✅
The Nitrogen Cycle: What Actually Kills Fish
Fish don't die from "overcrowding" visually—they die from ammonia poisoning. Here's the cycle:
- Fish waste + uneaten food → Ammonia (NH₃)
Toxic at 0.25ppm. Burns gills, causes rapid death. - Beneficial bacteria (Nitrosomonas) → Nitrite (NO₂)
Still toxic at 0.25ppm. Causes "brown blood disease" (methemoglobinemia). - More bacteria (Nitrobacter) → Nitrate (NO₃)
Mostly harmless up to 40ppm. Removed by water changes + plants.
Cycling establishes these bacteria. Without cycling, ammonia spikes to 4-8ppm within days and fish die gasping at the surface. This is why "New Tank Syndrome" kills 50% of beginner fish.
Filtration Capacity: The Safety Net
Your filter should turn over the tank volume 5-10x per hour. For a 30-gallon tank:
- Minimum: 150 GPH (gallons per hour)
- Ideal: 300 GPH
- Heavy bioload (goldfish, cichlids): 450+ GPH
Filter types ranked by effectiveness:
- Sump filter (best—massive bio-media, customizable, hides equipment)
- Canister filter (excellent—high GPH, lots of media, quiet)
- HOB (Hang-On-Back) (good—easy maintenance, decent bio-media)
- Sponge filter (basic—great for quarantine/fry tanks, gentle flow)
- Internal filter (trash—low GPH, tiny media, takes up tank space)
Pro tip: Run TWO filters. If one fails or gets clogged, the other maintains your cycle. Saved my tanks during a power outage once.
Species Compatibility: The Hidden Stocking Factor
You can stock correctly by numbers but still have a disaster if species don't mesh:
| Tank Mates | Compatibility | Why |
|---|---|---|
| Betta + Guppies | ❌ Bad | Betta mistakes guppy fins for rival betta, attacks |
| Goldfish + Tropical Fish | ❌ Terrible | Goldfish need 65-72°F, tropicals need 75-80°F |
| Angelfish + Neon Tetras | ⚠️ Risky | Angels eat tetras once they reach 4+ inches |
| Corydoras + Most Community Fish | ✅ Great | Peaceful bottom-dwellers, different niche |
| Rasboras + Tetras | ✅ Perfect | Similar size/temp, shoal together peacefully |
Maintenance Schedule Based on Stocking Level
Lightly stocked (50-70% capacity):
- Water changes: 25% every 2 weeks
- Gravel vacuum: Monthly
- Filter cleaning: Every 4-6 weeks
Moderately stocked (70-90% capacity):
- Water changes: 30% weekly
- Gravel vacuum: Every 2 weeks
- Filter cleaning: Every 3-4 weeks
Heavily stocked (90-100% capacity):
- Water changes: 40-50% weekly (sometimes twice)
- Gravel vacuum: Weekly
- Filter cleaning: Every 2 weeks
- Daily nitrate testing
Overstocked (>100%):
Don't. Just... don't. You're setting yourself up for constant water changes, disease outbreaks, and
fish deaths. I've tried it for high-bioload African cichlid tanks—it's exhausting and not worth it
unless you're a masochist.