QUICK ANSWER (30 Seconds)

Universal Formula: GPH = Tank Gallons × Turnover Rate

Turnover Rates by Bioload:

Light Bioload (planted tanks, few fish): 4x GPH
Medium Bioload (community tanks): 5-6x GPH
Heavy Bioload (goldfish, cichlids): 8-10x GPH

Example: 20-gallon community tank = 100-120 GPH filter

3-STEP FILTER SIZE CALCULATOR

STEP 1: Calculate Your Tank Volume

Length (inches) × Width (inches) × Height (inches) ÷ 231 = Gallons

Example: 24″ × 12″ × 16″ ÷ 231 = 19.9 gallons (≈20G)

STEP 2: Determine Your Bioload Level

LIGHT: Planted tank, 1-2 small fish, shrimp only
MEDIUM: Community tank, normal stocking (1 inch per gallon)
HEAVY: Goldfish, cichlids, overstocked, messy eaters

STEP 3: Apply GPH Formula

Light: Tank Gallons × 4 = Minimum GPH
Medium: Tank Gallons × 5-6 = Minimum GPH
Heavy: Tank Gallons × 8-10 = Minimum GPH

3 step flow chart

📊 COMPLETE FILTER SIZE CHART

Tank Size	Light Bioload GPH	Medium Bioload GPH	Heavy Bioload GPH
5 Gallon	20-25 GPH	25-30 GPH	40-50 GPH
10 Gallon	40-50 GPH	50-60 GPH	80-100 GPH
20 Gallon	80-100 GPH	100-120 GPH	160-200 GPH
30 Gallon	120-150 GPH	150-180 GPH	240-300 GPH
40 Gallon	160-200 GPH	200-240 GPH	320-400 GPH
55 Gallon	220-275 GPH	275-330 GPH	440-550 GPH
75 Gallon	300-375 GPH	375-450 GPH	600-750 GPH

⚠️ BIGGEST MISTAKE: Over-Filtering

65% of beginners buy filters that are TOO POWERFUL

Consequences:

❌ Excessive water current (fish stress, constant swimming)
❌ Plants get uprooted or damaged
❌ Betta fish can’t build bubble nests
❌ Shrimp get sucked into intake
❌ Wasted electricity ($20-40/year extra)

Real Example: 10G Betta tank with 200 GPH canister filter = Swimming in a hurricane

🎯 FILTER SIZE BY TANK TYPE

Betta Tanks (5-10 Gallons)

Recommended: Sponge Filter (20-50 GPH)

Why: Gentle flow, no strong current
GPH Range: 4x tank volume (5G = 20 GPH)
Best Products: Hikari Bacto-Surge ($12), Aquarium Co-Op ($15)
Michael’s Setup: 10G Betta with 40 GPH sponge filter (perfect flow)

Community Tanks (20-40 Gallons)

Recommended: HOB Filter (100-240 GPH)

Why: Easy maintenance, adjustable flow
GPH Range: 5-6x tank volume (20G = 100-120 GPH)
Best Products: AquaClear 50 (200 GPH, $40), Seachem Tidal 55 (240 GPH, $35)
Michael’s Setup: 20G community with AquaClear 30 (150 GPH)

Large Tanks (55+ Gallons)

Recommended: Canister Filter (300-750+ GPH)

Why: High capacity, quiet, powerful biological filtration
GPH Range: 5-6x for normal, 8-10x for cichlids
Best Products: Fluval 207 (300 GPH, $150), FX4 (700 GPH, $280)
Michael’s Setup: 75G cichlid with Fluval FX4 (700 GPH) + sponge backup

Goldfish Tanks (Any Size)

Recommended: 2x Filter Setup or High GPH

Why: Goldfish = heavy waste producers
GPH Range: 8-10x tank volume minimum
Best Strategy: HOB + Sponge combo OR oversized canister
Example: 30G goldfish = 240-300 GPH (AquaClear 70 + sponge)

💡 THE TRUTH ABOUT GPH RATINGS

Manufacturers LIE About GPH

Advertised vs Actual Flow Rates:

📦 Box Says: 300 GPH
🚰 Reality: 180-220 GPH (with media, height, tubing)
📉 Reduction: 25-40% loss from ideal conditions

Why? Rated GPH is tested with ZERO media, ZERO height, brand new impeller

Pro Tip: Buy filter rated for 1.5x your calculated GPH to account for media resistance

Example: Need 100 GPH → Buy 150 GPH rated filter → Actual output = ~105 GPH

🆚 ONE BIG FILTER vs TWO SMALL FILTERS

❌ 1 Large Filter

Cost: $150 (canister)
Redundancy: None (fails = disaster)
Maintenance: 60 min every 2 months
Flexibility: All-or-nothing flow

✅ 2 Small Filters

Cost: $65 (HOB + Sponge)
Redundancy: One fails = backup works
Maintenance: 15 min alternating monthly
Flexibility: Adjust each independently

Michael’s Recommendation for 40G+ Tanks: HOB + Sponge combo beats single canister

📋 REAL-WORLD EXAMPLES FROM MY TANKS

Example 1: 10G Betta Tank

Tank Volume: 10 gallons
Bioload: Light (1 betta + 5 shrimp)
Calculation: 10G × 4 = 40 GPH
Filter Used: Aquarium Co-Op Sponge (rated 50 GPH, actual ~40)
Result: Perfect gentle flow, betta happy for 3 years

Example 2: 20G Community Tank

Tank Volume: 20 gallons
Bioload: Medium (15 tetras + 6 corydoras)
Calculation: 20G × 5 = 100 GPH minimum
Filter Used: AquaClear 30 (rated 150 GPH, actual ~110)
Result: Crystal clear water, easy maintenance

Example 3: 40G Planted Tank

Tank Volume: 40 gallons
Bioload: Light (10 small fish, heavily planted)
Calculation: 40G × 4 = 160 GPH (light bioload + plants)
Filters Used: AquaClear 50 (100 GPH) + Sponge (60 GPH) = 160 total
Result: No algae, plants thriving, fish unstressed

Example 4: 55G Discus Tank

Tank Volume: 55 gallons
Bioload: Medium-High (6 adult discus)
Calculation: 55G × 6 = 330 GPH
Filter Used: Fluval 207 (rated 300 GPH, actual ~250) + spray bar to diffuse flow
Upgrade Needed: Added sponge filter (80 GPH) = 330 total

Example 5: 75G Cichlid Tank

Tank Volume: 75 gallons
Bioload: Heavy (15 African cichlids, overfed)
Calculation: 75G × 10 = 750 GPH (heavy waste)
Filter Used: Fluval FX4 (rated 700 GPH, actual ~550) + large sponge (150 GPH)
Result: Clean water despite heavy bioload, nitrates stable at 20 ppm

🚨 SIGNS YOUR FILTER IS TOO SMALL

⚠️ Water stays cloudy despite weekly water changes
⚠️ Ammonia or nitrite readings above 0 ppm
⚠️ Visible debris floating in water column
⚠️ Filter media clogs within 1-2 weeks
⚠️ Film forming on water surface

Solution: Add second filter OR upgrade to next size up

🚨 SIGNS YOUR FILTER IS TOO BIG

⚠️ Fish constantly swimming against current
⚠️ Plants getting uprooted
⚠️ Betta fish hiding behind decorations (avoiding flow)
⚠️ Food gets pushed into corners before fish can eat
⚠️ Excessive water splashing/noise

Solution: Add spray bar, baffle output, or reduce flow rate (adjustable filters)

🎓 ADVANCED TIPS FROM 12 YEARS EXPERIENCE

1. Account for Future Growth

Buy filter for final stocking level, not current. Adding 10 fish later? Size for that NOW.

2. Canister Filters: Subtract 30% from Rated GPH

Height, tubing, media = major flow reduction. 300 GPH canister ≈ 200 actual GPH.

3. Use Spray Bars for High GPH

Diffuses strong flow across tank surface. Essential for 300+ GPH filters.

4. Adjustable Flow is Worth $10 Extra

Filters like Seachem Tidal or AquaClear let you dial in perfect GPH.

5. Pre-Filter Sponges Save Fish

Covers HOB/canister intakes, prevents baby fish/shrimp suction deaths.

💰 COST COMPARISON: RIGHT SIZE vs WRONG SIZE

Scenario	Initial Cost	5-Year Cost	Fish Survival
Too Small (50 GPH for 20G)	$15	$120 (extra water changes, meds)	60% (cycle crashes)
Perfect Size (100-120 GPH for 20G)	$40	$80 (media replacement only)	95%
Too Big (300 GPH for 20G)	$150	$250 (electricity + stress deaths)	70% (stress, plants fail)

Conclusion: “Perfect size” saves $140 over 5 years AND keeps fish alive

📝 FILTER SIZING CHECKLIST

✅ Measure tank dimensions, calculate gallons
✅ Identify bioload level (light/medium/heavy)
✅ Calculate minimum GPH (gallons × turnover rate)
✅ Add 50% buffer for media resistance
✅ Check fish species flow preferences (bettas = low, hillstream loaches = high)
✅ Decide: single filter or combo (2 smaller = better redundancy)
✅ Verify filter type matches tank size (sponge <20G, HOB 20-40G, canister 40G+)
✅ Read actual user reviews for REAL GPH (ignore manufacturer claims)
✅ Plan for adjustable flow or spray bar if needed
✅ Budget for pre-filter sponges ($5) to protect small fish

🎯 BOTTOM LINE RECOMMENDATIONS

For Beginners:

Use the chart in this article (tank size + bioload = GPH range)
When in doubt, choose the MIDDLE of the GPH range
Adjustable flow filters = best insurance against mistakes

For Planted Tanks:

Use LOWER end of GPH range (4x turnover)
Strong flow = CO2 loss + plant damage
Prioritize gentle, even distribution

For Goldfish/Cichlids:

Use UPPER end of GPH range (8-10x turnover)
Consider 2 filters instead of 1 mega-filter
Clean mechanical media every 2 weeks