Protein skimmers are one of the few pieces of marine equipment where “bigger is always better” doesn’t hold. An undersized skimmer leaves organic waste in the water to break down into nitrate and phosphate, while one far above what the tank needs can pull out trace elements and coral food along with the waste. This guide sizes the skimmers IndieFur stocks by tank volume in litres, using the rated volume each maker publishes.
Nano and Small Tanks
Up to about 160 litres the closest fits are two Tunze hang-in skimmers that sit inside the display on a magnet holder, so no sump is needed. The Tunze DOC Skimmer 9001 is listed for 20 to 140 litres and the 9001 DC for 20 to 160 litres. For tanks with hard corals Tunze gives lower figures: up to 60 and 70 litres. Both are 110 x 63 x 215 mm, and the tank’s water level should sit at the surface mark on the body, so a top-up system helps.
Small to Medium Reef Systems
From about 150 to 400 litres, both in-tank and in-sump skimmers fit. The EHEIM Skim Marine 300 hangs inside the tank on a magnetic holder and is listed up to about 300 litres, and the sump-mounted Skim Marine 600 up to about 600. The Tunze DOC Skimmer 9004 DC hangs in the display and is listed for 60 to 300 litres depending on load. In the sump, the Deltec Skimmer 400i covers 100 to 400 litres and the Tunze DOC Skimmer 9404 up to 250 litres. Red Sea’s Reefer Skimmer RSK300 is rated 300 litres SPS-dominant and 600 litres mixed reef, with the DC Pump Upgrade Kit 300 available for hobbyists who want a variable-speed DC pump for finer control over skim wetness and air draw.
Skimmer for a 200 Litre Tank
A 200 litre system is inside the listed range of the Tunze 9004 DC, EHEIM Skim Marine 300, Tunze 9404 and Deltec 400i, and the Deltec Skimmer 600i range starts at 200 litres. Count the sump as well: a 200 litre display with a 60 litre sump is a 260 litre system, already past the 9404’s 250 litre rating. On a hard-coral reef the 9004 DC’s figure of up to 180 litres leaves no room, so the Red Sea RSK300 (300 litres even when SPS-dominant) or a Deltec fits better.
Skimmer for a 300 Litre Tank
A 300 litre system is at the top of the EHEIM 300 and Tunze 9004 DC ratings, so neither has headroom and a step up is the safer buy. The Deltec 400i covers it with room to spare, and the RSK300 is rated 300 litres SPS-dominant. On an SPS-dense reef the Aquavitro Division Skimmer 125 drops to 175 litres, so the Division Skimmer 250 (350 litres at that density) is the one to look at.
Medium to Large Reef Systems
Above 400 litres the skimmers sit in the sump, apart from the hang-in Tunze DOC Skimmer 9012 DC (200 to 1,200 litres depending on load). Red Sea names the RSK300, RSK600 and RSK900 after the litres they suit in an SPS-dominant system. The Red Sea Reefer Skimmer RSK600 is rated 600 litres SPS-dominant, 1,200 mixed reef and 1,800 fish-only (matching DC Pump Upgrade Kit 600), and the RSK900 900 litres SPS-dominant, with higher figures for other stocking (DC Pump Upgrade Kit 900). The Aquavitro Division 125 is listed up to 475 litres and the Division 250 up to 950. Deltec’s numbered range steps in bands that meet: the 1000i covers 600 to 1,000 litres, the 1500i 1,000 to 1,500 and the 2000i 1,500 to 2,000. Deltec’s bands meet end to end, which helps when your volume sits near a boundary.
Skimmer for a 1000 Litre Tank
A 1,000 litre system is the top of the Deltec 1000i range, so the 1500i gives more margin. The RSK600 is rated 1,200 litres for a mixed reef but 600 litres SPS-dominant, which is why the RSK900 (900 litres SPS-dominant, higher for other stocking) is the Red Sea model to look at, though a coral-dense 1,000 litre tank sits just above its SPS-dominant rating. The Aquavitro Division 250 is listed up to 950 litres, with a 1,000 litre figure at an average bioload, and the Tunze DOC Skimmer 9415 is listed up to 1,500 litres.
Tunze’s DOC Skimmer Range
Tunze’s DOC (Dissolved Organic Compound) skimmers cover an unusually wide span, from 20 to 3,500 litres, split between mains models and DC variants run from a controller. The 9404, 9415 and 9430 (up to 3,000 litres) are sump skimmers, and the 9430 DC is listed up to 3,500 litres with adjustable pump output. The DC variants add adjustable pump speed, which matters most when bioload changes (heavier fish stocking or coral growth) and you want to skim harder without replacing the unit.
Quick Reference
Rated volumes are the makers’ own figures from each product page, read on 15 October 2026.
| Skimmer | Type | Rated volume as the maker states it |
|---|---|---|
| Tunze DOC 9001 | Hang-in, mains | 20 to 140 L; up to 60 L with hard corals |
| Tunze DOC 9001 DC | Hang-in, DC controller | 20 to 160 L; up to 70 L with hard corals |
| Tunze DOC 9004 DC | Hang-in, DC controller | 60 to 300 L depending on load; up to 180 L with hard corals |
| Tunze DOC 9404 | In-sump, mains | Up to 250 L |
| EHEIM Skim Marine 300 | In-tank, magnetic holder | Up to about 300 L |
| Red Sea Reefer RSK300 | In-sump, AC pump (DC kit available) | 300 L SPS-dominant, 600 L mixed reef, 900 L fish-only |
| Deltec 400i | In-sump, DC pump | 100 to 400 L |
| Tunze DOC 9012 DC | Hang-in | 200 to 1,200 L depending on load |
| Aquavitro Division 125 | In-sump | 500 L average bioload (listed up to 475 L); 175 L extremely high SPS density |
| EHEIM Skim Marine 600 | In-sump | Up to about 600 L |
| Deltec 600i | In-sump, DC pump | 200 to 600 L |
| Red Sea Reefer RSK600 | In-sump, AC pump (DC kit available) | 600 L SPS-dominant, 1,200 L mixed reef, 1,800 L fish-only |
| Deltec 1000i | In-sump, DC pump | 600 to 1,000 L |
| Red Sea Reefer RSK900 | In-sump, AC pump (DC kit available) | 900 L SPS-dominant; higher for mixed reef and fish-only |
| Aquavitro Division 250 | In-sump | 1,000 L average bioload (listed up to 950 L); 350 L extremely high SPS density |
| Deltec 1500i | In-sump, DC pump | 1,000 to 1,500 L |
| Tunze DOC 9415 | In-sump, mains | Up to 1,500 L |
| Deltec 2000i | In-sump, DC pump | 1,500 to 2,000 L |
| Tunze DOC 9430 | In-sump, mains | Up to 3,000 L |
| Tunze DOC 9430 DC | DC controller | Up to 3,500 L |
How to Read a Skimmer’s Rated Volume
A rated volume is the maker’s figure, and makers present it differently. Red Sea gives SPS-dominant, mixed reef and fish-only ratings. Aquavitro gives an average-bioload figure and a lower one for extremely high SPS density. Deltec and EHEIM give a single range. Tunze rates against a tank stocked with leather corals and publishes a reduction table: 40% less for mixed leather and stony (LPS) corals, 60% less for predominantly SPS, and 70% less for SPS with a heavy fish population. Tunze’s example: a skimmer rated for 1,000 litres suits about 600, 400 or 300 litres in those cases.
Read the figure that matches what you keep, and count the whole system. Where only one number is given, treat it as the maker’s rating with no stocking adjustment, and go up a size if the tank is heavily stocked or fed heavily. Where litres and US gallons disagree, size against the litres.
How to Actually Size One
Match the skimmer’s rated tank size to your system’s total water volume, not just the display tank. Sump and refugium volume count toward total system size and change how much a skimmer needs to process. When your bioload sits between two size tiers, size up rather than down; a skimmer running slightly under capacity is easy to dial back with a valve or DC speed control, while one that’s genuinely undersized can’t be corrected without replacing it. Once the skimmer is sized, our marine aquarium accessories roundup covers other equipment worth planning for, including auto top-off, dosing, water movement and UV sterilization.
Protein Skimmer Sizing Calculator
Enter your total system volume (display + sump + refugium, as covered above) and your bioload. Skimmer ratings change significantly with bioload, so the same skimmer suits a much smaller SPS tank than a fish-only one. This uses each manufacturer’s own published rating for the bioload you pick.
Ratings are the manufacturers’ own published figures. Where a manufacturer publishes a single overall rating rather than per-bioload bands, that figure is used and flagged.
In-Sump vs Hang-On vs Internal Skimmers
In-sump skimmers stand in a chamber of the sump and are the only layout above 300 litres, apart from the Tunze 9012 DC. Each works within a water depth: Deltec 15 to 25 cm (16 to 26 cm for the 1500i and 2000i), Red Sea 16 to 20 cm for the RSK300. The Tunze 9404 tolerates 80 to 250 mm, and the 9415 and 9430 100 to 200 mm, with no setting to change, which suits a sump whose level moves. An auto top-off helps hold the level steady, and Deltec recommends one. Measure the cabinet too: the Deltec 400i stands 51 cm tall and needs 15 mm more to lift the cup out.
The smaller skimmers go inside the display instead. The Tunze 9001, 9001 DC, 9004 DC and 9012 DC and the EHEIM Skim Marine 300 sit in the tank on a magnet holder, which suits a tank with no sump. They need clear height above them to lift the cup out. None of the models covered here hangs on the back of the tank.
DC Pump or AC Pump
Red Sea’s RSK skimmers ship with an AC pump. The DC Pump Upgrade Kits (300, 600 and 900) swap in a variable-intensity PSK DC-1200 pump and add a skim sensor whose self-levelling feature adjusts pump intensity to prevent over-skimming and overflow. The kit works with the ReefBeat app and needs a ReefRun Dual Controller, sold separately; without it the pump will not run. Tunze’s DC models are run from a controller with adjustable output, and on the 9001 DC and 9004 DC its cup cleaning button pauses the pump for about ten minutes. Deltec’s skimmers all use a 24 V DC pump on a ten-stage controller with a feed pause button. The Tunze mains models are listed for 230 V / 50 Hz.
What a Skimmer Cannot Do
A skimmer removes dissolved organic waste before it breaks down into nitrate and phosphate. It does not remove nitrate and phosphate already in the water, replace water changes or provide biological filtration, and it does nothing in a freshwater tank. A bigger skimmer does not make up for overfeeding. In the sump it works alongside media, not instead of it, and our marine aquarium filter media guide sets out which media solves which problem.
Setting Up and Breaking In a New Skimmer
Assemble the skimmer, set the sump to the water level the maker lists and keep it there. The Red Sea pages say to turn the skim regulator fully to Minus before the first switch-on, and the Aquavitro page warns that closing the air intake screw too far can damage the pump’s needle impeller. Expect a rough start: the Red Sea pages call over-skimming in the first days normal and suggest stripping and washing the skimmer if it carries on past a week, and Deltec says a newly filled tank can give a flood of fine bubbles for two to three weeks. Change one setting at a time and judge it in days, not minutes.
Cleaning the Cup and Maintaining the Skimmer
Empty the cup as often as it fills, which depends on bioload and how wet the skimmer runs, and check it rather than working to a fixed schedule. Skimmate that backs up into the neck goes straight back into the tank. The Red Sea pages say to use the neck cleaner daily, which allows 7 to 10 days between washing the cup and wiper, because a sludge layer in the neck cuts foam production by 30 to 40%, and to clean the pump monthly. Deltec asks for a weekly check of the water level adjuster and a pump clean every six months. Rinse parts in fresh water. Red Sea says that if you wash the cup with detergent, it must be rinsed very thoroughly. If a skimmer stops foaming, check the neck, air path and pump before assuming a fault.
Common Skimmer Mistakes
The usual errors are sizing against the display tank and forgetting the sump, buying at the very top of a rated range, and judging a new skimmer in its first week. A sump level that drifts through the day is another; Red Sea notes a skimmer cannot hold a foam point in a moving level. Red Sea notes that foods and supplements are surface-active and can upset foam production.
Protein Skimmer FAQ
What size protein skimmer do I need for a 200 litre tank?
Pick a skimmer whose rated volume covers the whole system, display plus sump, with margin. The Deltec 400i and 600i, EHEIM Skim Marine 300 and Red Sea RSK300 all list ranges that include 200 litres.
What size protein skimmer do I need for a 300 litre tank?
The EHEIM Skim Marine 300 and Tunze 9004 DC top out at 300 litres, so go one size up to the Deltec 400i or Red Sea RSK300. For an SPS-dense tank, check the Aquavitro Division 250.
Do I need a protein skimmer on a fish-only tank?
Not as a fixed rule. Our protein skimmer glossary page notes some nano tanks run without one. It makes more sense as stocking and feeding rise. Red Sea rates the RSK300 for 900 litres fish-only but 300 SPS-dominant.
How often should I clean a protein skimmer?
Empty the cup as often as it fills, and clean the neck, pump and other parts on the maker’s schedule.
Is a bigger protein skimmer always better?
No. A skimmer far above the system takes more cabinet room (the Deltec 2000i footprint is 300 x 225 mm against 205 x 120 mm for the 400i) and can pull out trace elements and coral food along with the waste. Models with a valve or DC speed control can be turned down; an undersized one cannot be turned up past its rating.
