My rule for any filter, canister, internal, hang-on or sponge: pick one rated at 2.5 times the water volume, minimum. The ideal is 5 to 8 times. The better the filtration, the better the biology. Customers often buy filters rated at only 1x or 1.5x their tank volume.
This page is about internal (submersible) filters. They sit inside the tank, and the specs below come from each manufacturer’s published sheets. To see how they work mechanically, and when to pick one over a canister or hang-on-back, read our guide to internal aquarium filters explained. Comparing external canisters? Try the canister filter comparison guide. For outdoor ponds, there is a pond filtration guide.
For the three filtration stages and how flow ties in, read our aquarium filtration guide alongside this page.
How internal filtration works
An internal filter does up to three jobs at once:
- Mechanical filtration uses a foam or sponge stage to trap debris, waste and uneaten food.
- Biological filtration relies on beneficial bacteria that colonize the media and convert ammonia and nitrite into far less toxic nitrate, as part of the aquarium’s nitrogen cycle.
- Chemical filtration, on some models, uses activated carbon or similar media to remove dissolved impurities and odors. It is usually an optional swap-in cartridge rather than a permanent stage.
Internal filters sit fully submerged, so they are compact and easy to install. The tradeoff: a smaller media chamber than an equivalent external canister. Know that before you buy.
Matching flow rate to tank size
Past the 2.5x minimum, check turnover: how many times per hour the filter circulates your total tank volume. Aim for roughly 5 to 8 times. A filter rated at 500 L/h suits a tank of roughly 62 to 100 litres at that rate. The largest tank size printed on the box is often set by media and bioload capacity instead, so it can run higher.
| Model | Flow Rate | Tank Size for 5 to 8x Turnover |
|---|---|---|
| Fluval U1 | 250 L/h | ~31 to 50L |
| Fluval U2 | 400 L/h | ~50 to 80L |
| Sicce Shark Pro 500 | 500 L/h | ~62 to 100L |
| Sicce Shark Pro 700 | 700 L/h | ~87 to 140L |
| Sicce Shark Pro 900 | 900 L/h | ~112 to 180L |
Manufacturer maximum-tank-size ratings usually sit higher than the turnover math above. Normal. It reflects filtration capacity, not a promise of 5 to 8x circulation at the top of the range, and the two numbers are not meant to match. Running a heavily planted tank, or fish with a high bioload? Size toward the lower half of the filter’s rated range.
IndieFur’s internal filters compared
Here is our internal filter range, by brand and series.
| Model | Flow Rate | Mfr. Tank Rating |
|---|---|---|
| ISTA Ceramic Internal Filter | Air-driven (no motor L/h) | Nano/compact (7×7×12cm unit) |
| Fluval U1 | 250 L/h | Up to 55L |
| Fluval U2 | 400 L/h | Up to 110L |
| Fluval U4 | Not clearly published* | 130 to 240L |
| Sicce Shark Pro 500 | 500 L/h | Not published by Sicce |
| Sicce Shark Pro 700 | 700 L/h | Not published by Sicce |
| Sicce Shark Pro 900 | 900 L/h | Not published by Sicce |
| Eheim Pick Up 45 | Not published | Smallest in the Pick Up series** |
| Eheim Pick Up 60 | Not published | Up to 60L |
| Eheim Pick Up 160 | Not published | Up to 160L |
| Eheim Pick Up 200 | Not published | Up to 200L |
*Fluval’s own spec sheet for the U4 lists a garbled output figure instead of a clean L/h number, so we left it out. The 130 to 240L tank rating is confirmed from Fluval’s published specs.
**The Pick Up 45 listing description repeats the 160L spec from the Pick Up 160 in the same series. It is the smallest model in a 45/60/160/200 series, so treat it as rated for smaller tanks than the 60, not 160.
One more item in this category, the Sicce Shark MINI SKIM 350, is a surface skimmer rather than a submerged internal filter. Do not compare it with the models above on flow rate.
Nano and small tanks: the ISTA Ceramic option
The ISTA Ceramic Internal Filter runs off an air pump instead of a motor, so there is no published L/h figure. At 7×7×12cm it fits nano and pico tanks, where even the Fluval U1’s 250 L/h would be too much current. A fair start for a small shrimp or betta tank.
Why biological filtration matters most
Mechanical filtration is the job you can see. Easy. Biological filtration keeps fish alive long-term. Beneficial bacteria convert ammonia and nitrite, both toxic to fish even at low concentrations, into nitrate. Nitrate is far less harmful, and regular partial water changes remove it. Without enough space for that colony to establish, fish can show stress, poor growth and higher disease risk, even in a tank that looks perfectly clean to the eye and tests fine for a week or two. Then it turns. A larger media chamber generally supports a steadier colony. One reason the Sicce Shark Pro and the Fluval U-series, with dedicated media cartridges, tend to hold water parameters more steadily than very compact nano filters.
Maintenance, noise, and running costs
Some things only show up months after you buy.
- Cleaning frequency: most internal filters need the media rinsed every 2 to 4 weeks. Do not scrub biological media hard, since that is where your bacterial colony lives.
- Noise: motorized internal filters (Fluval U-series, Sicce Shark Pro/ADV) run quieter than air-driven designs but are not silent. Placing the unit against a solid tank wall reduces vibration noise.
- Running cost: internal filters are low-wattage compared to canisters, so electricity is rarely a deciding factor. Consumable media cartridges, such as Fluval’s included Poly/Carbon cartridges, add a small recurring cost.
Common mistakes to avoid
- Buying a filter rated at 1x or 1.5x the tank volume. The minimum is 2.5x, and 5 to 8x is ideal.
- Buying on price alone, without checking the flow-rate and tank-size match. Too small, and the filter won’t keep up with the bioload. Too big, and it can create too much current for some fish.
- Ignoring biological filtration capacity because the box only advertises strong mechanical cleaning, which is the part you can see.
- Choosing high flow for sensitive species. Bettas, shrimp and some nano fish do poorly in strong current. Rating does not change that.
- Assuming internal always beats external. For larger or heavily stocked tanks, an external canister usually has more media capacity than any internal unit. See our canister filter comparison guide if you are in that range.
Buying guide by tank size
Nano/pico tanks, shrimp, or betta bowls:
The ISTA Ceramic or the Fluval U1 (~31 to 50L at 5 to 8x turnover) both suit gentle, low-current setups.
Small to medium community tanks:
The Eheim Pick Up 60 or 160, or the Sicce Shark Pro 500 (~62 to 100L at 5 to 8x turnover), fit most standard small-to-medium freshwater tanks.
Larger or higher-bioload tanks:
The Sicce Shark Pro 900 (~112 to 180L at 5 to 8x turnover), Eheim Pick Up 200 (up to 200L) or Fluval U4 (130 to 240L) handle bigger tanks, or heavier stocking like goldfish or cichlids.
Tanks larger than ~250L, or very heavy bioload:
Go external. Our canister filter comparison guide covers models with more media capacity than any internal filter offers.
Setting up an outdoor pond instead?
Pond filters work at a different scale entirely. Our guide to choosing the right filtration system for a healthy fish pond has pond-specific models and sizing.
Frequently asked questions
How often should I clean an internal filter?
Rinse the media every 2 to 4 weeks, depending on fish load and tank size. Do it in removed tank water. Not tap water. Chlorine in tap water can kill off the beneficial bacteria you are trying to protect.
Are internal filters suitable for large aquariums?
They work best for small and medium tanks. Above roughly 200 to 250L, or in heavily stocked tanks, an external canister generally offers more media capacity. See our canister filter comparison guide.
What size filter should I buy for my aquarium?
Rated at 2.5 times your water volume at minimum, and 5 to 8 times is ideal. Divide the filter’s rated L/h by 5 to 8 to estimate its ideal tank size, instead of relying only on the manufacturer’s maximum-tank-size figure (see the turnover table above).
What’s the difference between mechanical and biological filtration?
Mechanical filtration removes visible debris and particles. Biological filtration uses beneficial bacteria to break down invisible, toxic ammonia and nitrite into safer nitrate. Slow to notice. It decides whether the fish do well long-term.
Can a filter improve fish health directly?
Yes. Proper filtration removes waste, supports the bacteria that process toxins, and keeps water parameters stable. All of that reduces stress and disease risk in fish.
About this guide
This guide was compiled and reviewed by the IndieFur content team using each manufacturer’s published product specifications (Fluval, Sicce, EHEIM, ISTA).
