Internal & Submersible Filters: When to Use Them

Internal & Submersible Filters: When to Use Them | Aqualista

💧 Internal (submersible) filters are compact, all‑in‑one units that sit inside the aquarium — This guide covers when to use them, their advantages and disadvantages, and best practices for setup and maintenance.

What Is an Internal (Submersible) Filter?

An internal filter is a fully submersible unit that sits inside the aquarium, usually attached to the glass via suction cups. It contains a pump, intake strainer, and media compartment (sponge, bio‑media, carbon). Water is drawn in and expelled through an adjustable nozzle.

Internal filters are often used as budget‑friendly options for small to medium tanks (5‑30 gallons). Aquarium Co‑Op notes that they are simple to install and require no external plumbing. They are popular for quarantine tanks, hospital tanks, and temporary setups because they can be moved easily. However, they take up space inside the tank and are less powerful than HOB or canister filters. For a general filter overview, see our main filter guide.

Pros and Cons of Internal Filters

Pros: cheap, easy to install, no external hoses, quiet (submerged), can be aimed, good for small tanks. Cons: take up tank space, limited media capacity, less mechanical filtration than HOB, outflow can be strong for small fish, pump may overheat if exposed.

Internal filters are a mixed bag. Pros: Prices start at $10‑20. Installation is just suction cups and plugging in. They are quiet because the motor is underwater. The nozzle can be adjusted to point in any direction, allowing targeted flow. Cons: They are bulky and hide behind decor. Media capacity is small (often just a thin sponge). They do not polish water as well as a HOB. The outflow can be too strong for bettas or small fry without a baffle. If the water level drops below the pump, it can overheat. For a comparison with other types, see our filter type comparison.

✅ AdvantagesCheap, easy to install, adjustable output, no external parts, good for small or quarantine tanks

❌ DisadvantagesTakes interior space, limited media capacity, may create strong current, pump can overheat if water level drops, less biological capacity

When Should You Use an Internal Filter?

Best for: small tanks (under 20 gallons), quarantine/hospital tanks, temporary setups, tanks with limited rim space (e.g., rimless or curved), and as supplemental filtration for dead spots.

Internal filters are not the best primary filter for most tanks but excel in specific situations. Quarantine/Hospital tanks: Easy to move and clean, no external parts to disinfect. Small nano tanks (5‑10g): An internal filter can be sufficient for light stocking. Rimless or curved tanks: HOB filters may not fit; internal filters are a good alternative. Supplemental filtration: Place an internal filter in a dead spot to increase circulation. For a heavily stocked 20‑gallon tank, a HOB or canister is better. For more guidance, see our internal filter use cases.

🔍 Good applications for internal filters – Quarantine tanks (easy to disinfect)
– Hospital tanks (no external hoses)
– Nano tanks (5‑10g with light bioload)
– Rimless / bowfront tanks (no HOB rim)
– Spot circulation (dead zones)

How to Choose the Right Internal Filter Size (Flow Rate)

Aim for 4‑6 times turnover per hour. For a 10‑gallon tank, choose a filter rated 40‑60 GPH. For bettas or shrimp, lower flow (30‑40 GPH) is better; for goldfish, higher (80‑120 GPH). Many internal filters have adjustable flow.

Flow rate is often adjustable. Aquarium Co‑Op recommends matching the filter to your tank volume. For a 10‑gallon betta tank, a 40 GPH internal filter at its lowest setting works. For a 20‑gallon community tank, 80‑120 GPH is adequate. Over‑filtering with an internal is safe, but the outflow may be too strong – use a baffle or aim it at the glass. Most internal filters come with a spray bar or directional nozzle. For a detailed selection guide, see our filter sizing guide.

📊 Internal filter flow recommendations – 5‑10g (betta, shrimp): 30‑50 GPH (use lowest setting)
– 10‑20g community: 50‑100 GPH
– 20‑30g community: 100‑150 GPH
– For higher bioload, use two units or a larger filter

Setting Up an Internal Filter (Step‑by‑Step)

Rinse the media, attach suction cups, place the filter in the tank (fully submerged), aim the output nozzle, plug in. Ensure the water level never drops below the pump intake. Install pre‑filter sponge on intake if needed.

Installation is straightforward. Step 1: Rinse the sponge and any other media in dechlorinated water. Step 2: Attach suction cups to the filter body. Step 3: Place the filter near the bottom or mid‑tank, ensuring the intake is fully submerged. Step 4: Adjust the output nozzle or spray bar direction. Step 5: Plug the filter into an outlet. Step 6: If the flow is too strong for your fish, point the nozzle toward the glass or add a sponge on the outflow. Step 7: For fry or shrimp, add a pre‑filter sponge over the intake to prevent tiny animals from being sucked in. For more details, see our internal filter setup guide.

🧪 Important: Never run an internal filter with the pump exposed to air. Water cools the motor; running dry can cause overheating and damage. Keep the water level above the pump.

Maintaining an Internal Filter (Cleaning Schedule)

Clean the sponge every 2‑4 weeks by squeezing in removed tank water. Clean the impeller and intake every 2‑3 months. Replace chemical media monthly. Do not use tap water. Never replace all biological media at once.

Internal filters require similar maintenance to other filters. Sponge cleaning: Turn off and unplug. Remove the sponge and squeeze in a bucket of tank water. Rinse the impeller well. Impeller and intake: Remove the impeller cover, lift out the impeller, and rinse any debris. Use a small brush for the intake tube. Media replacement: If the filter uses carbon cartridges, replace monthly. For better biofiltration, replace the carbon with ceramic rings in a mesh bag. Never clean the biological media with tap water. For a full maintenance guide, see our filter maintenance hub.

🧽 Pro tip: Keep a spare sponge inside the tank (just drop it in) for 3‑4 weeks. Then you can swap it with the dirty sponge – instant cycled filter!

Troubleshooting Common Internal Filter Problems

Low flow: clean sponge and impeller. No flow: check if pump is clogged or impeller stuck; clean. Noise: air trapped – tilt the filter to release air. Overheating: water level too low – add water.

Low flow or no flow: First, unplug and check the intake for debris. Remove the sponge – if it is heavily clogged, it restricts flow. Clean the sponge and impeller. Grinding noise: Usually debris in the impeller well. Remove the impeller cover and clean. Air bubbles: If the filter spits air, the water level may be too low, or air is trapped in the pump. Tilt the filter underwater to release air. Filter gets hot: The pump is running dry – immediately add water to the tank. If the filter does not restart after cleaning, the impeller may be broken – replace it. For more, see our filter troubleshooting guide.

🔧 Quick fixes for internal filters – Low flow: clean sponge and impeller
– Noise: tilt to release trapped air
– Not starting: check impeller for debris, ensure water level
– Overheating: add water, check for blockage

Best Internal Filter Models for Small Tanks

Popular models: Fluval U series (adjustable flow, media baskets), AquaClear Power Filter (internal version), Eheim internal filters (compact, quiet), and budget brands like Aqueon QuietFlow. For nano tanks, the Azoo Mignon is popular.

Fluval U series (U1, U2, U3): Adjustable flow, multi‑stage media baskets, quiet. AquaClear Power Filter (internal): Similar media basket design to the HOB. Eheim internal filters (e.g., Eheim aquaCor): Very quiet, German quality. Aqueon QuietFlow internal: Budget option with replaceable cartridges. For a 10‑gallon betta tank, the Fluval U1 on low setting works well. For a 20‑gallon community, a Fluval U2 is adequate. For up‑to‑date recommendations, see our internal filter review.

📋 Recommended internal filter products (nofollow):
• Fluval U Series internal filters
• Eheim internal filters
• AquaClear internal power filter
• Pre‑filter sponge (for intake)

📚 Explore more filter resources: ← Main Filter Hub (T1) You are here Filter type comparison (T3) HOB filters (T2) Canister filters (T2) Sponge filters (T2) Filter maintenance (T2) Filter media (T2)

📖 References & further reading
• Aquarium Co‑Op – Internal filter guide
• Aquarium Co‑Op – Filter sizing
• Merck Veterinary Manual – Submersible filtration
• Fluval U series official page