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If you are new to fishkeeping, choosing the right filter size can feel confusing. You see gallons per hour on boxes, different filter types on shelves, and many opinions online. The good news is that you can select the right filter with a few simple rules. In this guide, you will learn how to calculate the filter size you need, how to adjust for your fish and plants, and how to avoid common mistakes that cost money and stress your fish.
Why Filter Size Matters
Your filter does three important jobs. It traps debris so your water looks clear. It grows helpful bacteria that remove toxic ammonia and nitrite. It moves water and creates surface agitation so oxygen can enter the tank and carbon dioxide can leave. If your filter is too small, waste builds up and water can become dangerous. If your filter is too strong, fish can struggle in the current or plants can uproot. The goal is to match filter size to your tank’s needs so the ecosystem stays stable and easy to maintain.
Key Terms You Must Know
Tank Volume and Bio-Load
Tank volume is the actual amount of water in your aquarium. The glass size may be 20 gallons, but hardscape, substrate, and equipment reduce water volume by 10 to 20 percent. Bio-load is how much waste your fish and invertebrates produce. Many small fish create less waste than a few large messy fish like goldfish. Higher bio-load needs more filtration and more water changes.
Flow Rate and Turnover
Flow rate is how much water your filter can move per hour. In the United States, you see this measured as GPH, which means gallons per hour. In many other places you see LPH, liters per hour. Turnover means how many times your filter can move the entire tank volume in one hour. If your 20-gallon tank has a 200 GPH filter, the turnover is 10 times per hour. You can use either unit. One gallon is about 3.785 liters. To convert GPH to LPH, multiply by 3.785.
Media Capacity and Stages
Media capacity is how much sponge, ceramic, or biological media the filter can hold. More media means more space for beneficial bacteria and more stable water. Filter stages are mechanical, biological, and sometimes chemical. Mechanical catches debris. Biological holds bacteria that convert ammonia and nitrite. Chemical media like carbon can polish water or remove odors. Two filters can have the same flow rate but very different media capacity. The one with more media usually supports healthier bacteria and safer water.
Head Height and Flow Loss
Manufacturers test flow with no pipe length, no lift, and clean media. In real aquariums, hoses, elbows, height, and dirty media reduce flow by 25 to 50 percent. This is called head loss. Canister filters and sump pumps are most affected. If you calculate that you need 300 GPH, you may want to choose a model rated for 400 to 500 GPH so the real flow matches your target after losses.
Surface Agitation and Oxygen
Fish and bacteria need oxygen. Gentle surface ripples increase gas exchange. Strong agitation is important for tanks with high bio-load, warm water, or no plants. Planted tanks with injected CO2 often use softer surface movement to hold CO2 in the water. Choose your flow and outlet style to match your goals.
The Simple Sizing Rule
The 4 to 10 Times Rule
As a simple rule, aim for a filter that turns your tank volume over 4 to 10 times per hour. For example, a 20-gallon tank needs 80 to 200 GPH. Use the low end for lightly stocked, gentle-flow fish or planted tanks. Use the high end for messy fish, crowded tanks, or bare tanks with little plant help. This rule gives you a quick target that works for most freshwater aquariums.
Freshwater and Saltwater Targets
In freshwater community tanks, 5 to 8 times per hour usually works well. In planted aquariums with lots of plants and delicate fish, 4 to 6 times per hour is often enough. In saltwater fish-only systems, aim higher, around 8 to 12 times per hour, because marine fish and live rock benefit from strong movement and oxygenation. If you have a reef tank, you still need a filter, but water movement inside the tank from wavemakers often handles most flow. In that case, your return pump and protein skimmer do the heavy lifting for gas exchange and nutrient export.
Stocking and Species Adjustments
Goldfish, cichlids, and large plecos create a lot of waste. For them, target the higher end of the range or even more. Bettas, nano fish, and shrimp prefer gentle flow, so stay near the lower end and spread flow using a sponge filter, spray bar, or baffle. Always watch your fish. If they are clinging to decor to escape the current, reduce the flow or redirect the output.
Plant Level and Aquascapes
Heavily planted aquariums process ammonia naturally and can be run at modest turnover. However, plants also trap debris, so mechanical filtration is still important. If you inject CO2, avoid strong surface agitation to limit CO2 loss. Use a lily pipe or spray bar to create even, gentle flow across the plants. If you do not inject CO2, a little extra surface movement will improve oxygen and help prevent algae.
Filter Types and How They Affect Sizing
Sponge Filters
Sponge filters are air-driven and gentle. They are perfect for bettas, shrimp, fry, quarantine tanks, and hospital tanks. They do excellent biological filtration and safe mechanical filtration. Flow is lower than a power filter, so you may use a larger sponge or two sponges for better turnover. A good air pump can run multiple sponges in small tanks. For anything above 20 gallons, consider pairing a sponge with another filter type.
Internal Power Filters
Internal filters sit inside the tank and are easy to install. They offer decent mechanical and some biological filtration. They are popular in small aquariums where hang-on-back units do not fit. Because they have limited media capacity, choose models with extra sponge or ceramic media space when possible. They can be a good upgrade from a basic sponge while keeping gentle flow.
Hang-On-Back Filters
Hang-on-back filters balance ease, cost, and performance. They are simple to maintain and can hold good media volume. Choose models that use reusable sponges and baskets rather than disposable cartridges. For most small to mid tanks, a HOB filter sized for one step larger than your tank is a safe bet. Two smaller HOBs at opposite ends of a long tank can give better circulation and redundancy.
Canister Filters
Canister filters sit under the tank and push water through large baskets of media. They have high media capacity and are quiet, which makes them great for larger tanks and demanding fish. Account for head loss and flow reduction as media clogs. Choose a model rated well above your calculated need and use a spray bar or directional outlet to tune flow. Clean prefilter sponges often to maintain performance.
Undergravel Filters
Undergravel filters pull water through the substrate using air or powerheads. They offer biological filtration across the whole bottom of the tank. They are simple but can trap debris in the gravel if you do not vacuum it often. Modern aquarists use them less because canisters and HOBs offer more flexible media options. If you choose undergravel, pair it with a HOB or sponge for better mechanical filtration.
Sumps and Wet or Dry Filters
A sump is an extra tank under your main tank with a return pump. Water overflows to the sump, passes through filter socks, sponges, or bio-media, then returns to the tank. Sumps add water volume and stability and are common in marine and large freshwater setups. Wet or dry trickle filters excel at oxygenating water and supporting bacteria. Size your return pump to reach your turnover target after head losses, and match your overflow rating to handle the flow safely.
Protein Skimmers for Marine Tanks
Skimmers are not filters in the freshwater sense, but they remove dissolved organics and improve oxygenation in saltwater tanks. They do not replace mechanical or biological filtration. Choose a skimmer rated for your tank’s real volume and bio-load, and pair it with a good return pump and flow inside the display.
Step by Step: Choose the Right Size
Step 1. Find your actual water volume. If your tank is 29 gallons and full of wood and rocks, assume about 25 gallons of water. If you use liters, do the same adjustment.
Step 2. Pick a turnover target. For a typical community freshwater tank, choose 5 to 8 times per hour. For a planted CO2 tank, choose 4 to 6. For goldfish or cichlids, choose 8 to 10. For saltwater fish-only, choose around 10, and ensure strong water movement from powerheads.
Step 3. Multiply your water volume by the turnover target to get your required flow. A 25-gallon water volume at 8 times per hour needs 200 GPH. In liters, multiply volume in liters by your chosen turnover to get LPH.
Step 4. Add a safety margin for head loss and clogging. Increase your required flow by 30 to 50 percent when choosing the filter model. For the 200 GPH need, look at a filter rated around 260 to 300 GPH.
Step 5. Consider media capacity and maintenance. If two filters have the same rated flow, choose the one with larger baskets and reusable sponges. More media is more forgiving, especially for beginners.
Step 6. Match flow style to your fish. If you keep bettas or long-finned fish, use a spray bar, baffle, or sponge filter to soften the current. For active fish that love flow, aim outlets along the length of the tank and create a circular pattern.
Step 7. Plan for redundancy if possible. Two moderate filters can be better than one big filter. You can clean one while the other keeps bacteria safe. If one fails, your fish are still protected.
Common Tank Scenarios with Recommendations
5 to 10 Gallon Betta or Shrimp Tank
Target 4 to 6 turnovers per hour. Gentle flow is important. A single large sponge filter with a decent air pump works well and keeps baby shrimp safe. An internal filter on the lowest setting is also fine if you add a prefilter sponge on the intake. Aim for 40 to 60 GPH on a 10-gallon, and adjust the outlet to reduce turbulence.
20 Gallon Community Tank
Target 5 to 8 turnovers per hour. A hang-on-back filter rated for 200 to 300 GPH is a good fit because real flow will settle around your target. Choose a model with sponge and ceramic media. If the current is too strong, add a baffle or a small sponge on the intake. Keep the outlet near the surface for oxygen.
29 to 40 Gallon Planted Tank
Target 4 to 6 turnovers per hour. A quiet canister filter with a spray bar is ideal. Aim the spray bar along the back wall to create a gentle circular flow that moves across plant leaves. If you inject CO2, reduce surface ripple. Ensure you have enough mechanical media to catch plant debris, and clean prefilters regularly.
55 Gallon African Cichlid Tank
Target 8 to 10 turnovers per hour because cichlids are active and can be messy. A canister filter rated for 350 to 500 GPH after head loss is a strong choice. Many keepers also add a second filter or a HOB to help with mechanical debris. Structure the rockwork so fish have shelter but water still moves through gaps.
75 Gallon Goldfish Tank
Target 10 turnovers per hour or more. Goldfish produce heavy waste. Pair a large canister with a second HOB or a second canister. Use generous mechanical media to catch waste and keep ammonia controlled. Strong surface agitation helps oxygenation, which goldfish love. Keep up on maintenance so flow does not drop suddenly.
125 Gallon Heavily Stocked Tank
Target 8 to 10 turnovers per hour or more, with redundancy. Two large canisters or a sump with a return pump are common solutions. If you use canisters, choose models with big baskets and coarse-to-fine mechanical stages. If you use a sump, choose a return pump that meets your target after head height and plumbing losses and add internal powerheads for in-tank circulation.
Reef Tank Example
For a 75 gallon reef, the filter’s turnover is only part of the picture. Aim for a return pump that gives 5 to 7 times the display volume after head loss, and add wavemakers to reach total in-tank flow of 20 to 40 times per hour, depending on coral type. Use a protein skimmer rated for the system volume. Even in reef systems, mechanical filtration and regular maintenance keep water clear and stable.
Avoid These Sizing Mistakes
Do not trust box ratings without thinking. Ratings are often for empty, clean filters at zero head height. In real life, flow drops. Build in a margin. Do not choose flow only and ignore media space. A compact filter with high flow but tiny media will clog fast and lose biological capacity. Do not blast delicate fish with current. If your filter is strong, diffuse the outlet with a spray bar or use a prefilter sponge to tame flow. Do not rely on chemical media to fix poor sizing. Chemical media can polish water, but it does not replace good mechanical and biological filtration or regular water changes. Do not clean all media at once. If you rinse everything in tap water, you kill beneficial bacteria and cause ammonia spikes.
Fine Tuning Flow Without Changing Filter
You can adjust your existing filter to match your fish. Use a spray bar to spread flow along the tank. Aim the outlet at the glass to reduce direct blast. Add a prefilter sponge to the intake. This protects shrimp and fry, increases mechanical filtration, and slightly reduces flow to a gentle level. Install a flow control valve if your model supports it, but do not restrict the intake or run the motor dry. Move the outlet near the surface for more oxygen, or lower it to reduce agitation in planted CO2 tanks. You can position two outlets on opposite ends to create a smooth circular pattern.
Maintenance Matters as Much as Size
A perfectly sized filter will still fail if you never clean it. Rinse mechanical sponges in old tank water weekly or biweekly so flow stays strong. Clean prefilters more often because they collect debris quickly. Rotate biological media cleaning so you never wash all media at once. Always use dechlorinated water for rinsing media so bacteria survive. Replace disposable cartridges with reusable sponges and ceramic media to save money and keep bacteria stable. Watch your flow. If the waterfall weakens or the spray bar loses force, it is time to clean. Stable flow equals stable oxygen and safer fish.
Budget, Noise, and Energy Use
Filters vary in price. Sponge and HOB filters are budget friendly. Canisters and sumps cost more but offer quiet operation and higher media capacity. Look at long-term costs too. Reusable media costs less than cartridges. Larger pumps use more power, and power use adds up over time. If your tank is in a bedroom or living room, prioritize quiet models and rubber feet to reduce vibration. Good sizing reduces how hard your filter has to work and helps keep noise down.
How to Calculate for Odd Shaped Tanks
If your tank is a bowfront, hexagon, or custom, measure water volume by length times width times height in inches, divided by 231 to get gallons. For liters, measure in centimeters, multiply length, width, and height, and divide by 1000. Then subtract a bit for substrate and decor. Use that number for your turnover calculation, then choose a filter with a margin for real-world losses. If math is not your thing, use an online volume calculator and then apply the same turnover rule.
Extra Tips for Special Cases
Turtle and Axolotl Tanks
These animals produce heavy waste and often live in water without plants. Choose filtration far above fish standards, often 10 to 15 turnovers per hour with lots of mechanical media. Combine a large canister with frequent cleaning. Watch temperature and oxygen, as warm, dirty water becomes dangerous fast.
Fry and Breeding Setups
Use sponge filters to protect tiny fish from getting sucked in. Gentle flow helps fry find food. You can stack two sponges for more surface area. Maintain water changes to keep nitrates low without harsh flow.
Quarantine and Hospital Tanks
Keep it simple with a cycled sponge filter. Avoid carbon while using medication because carbon can remove meds. After treatment, add carbon if you want to clear the water, then remove it again.
FAQ Quick Answers
Can a filter be too big?
Yes, if the flow is too strong for your fish. However, you can use a larger filter body for media capacity and then reduce flow with a spray bar or control. Many keepers choose a bigger filter for media and adjust the output to keep fish comfortable.
Do I need two filters?
No, but two filters add safety. You can maintain one while the other keeps bacteria safe, and if one fails, the other keeps water moving. This is helpful on longer tanks where one filter might not reach all areas.
How often should I clean my filter?
Clean mechanical media every one to two weeks, depending on debris. Rinse biological media gently when flow drops, but never all at once. Always use tank water or dechlorinated water for rinsing to protect bacteria.
Is chemical media required?
No. Mechanical and biological media are essential. Chemical media like carbon is optional and useful for removing odors, tannins, or medications. Focus your budget on good sponges and ceramics first.
What about air stones and oxygen?
An air stone increases surface agitation and oxygen. It is especially helpful in warm water, during medication, or if your filter flow is gentle. A filter plus an air stone can be a strong combination for bio-load and oxygen needs.
Putting It All Together
To choose your filter size, find your real water volume, pick a turnover target that fits your fish and plants, and then select a filter that reaches that flow after real-world losses. Prefer models with large, reusable media and plan simple maintenance to keep flow steady. Adjust the outlet so your fish are comfortable, and add redundancy if your budget allows. When in doubt, choose a filter with more media capacity and then soften the flow with a spray bar or prefilter sponge. With these steps, your fish tank will be clear, stable, and easy to enjoy.
Conclusion
The right filter size is not a mystery. Most freshwater tanks do well with 4 to 10 turnovers per hour, adjusted for species, planting, and stocking. Pick a filter with enough media, allow for flow losses, and maintain it regularly. If your fish prefer calm water, diffuse the flow. If your fish are messy, increase turnover and mechanical filtration. By following these simple rules, you protect your fish, simplify your routine, and build a healthy aquarium that looks great day after day.