A hang-on-back filter, also referred to as a H.O.B or hang-on filter, has often been a common choice for those looking for an easy-to-use external filter that doesn’t take up too much space. The Marina Slim filter range has mastered the art of not taking up too much space while still having  a good volume of filter media storage. It comes in three sizes, the S10, S15 and S20 for aquariums up to 10, 15 and 20 gallons respectively.

High and adjustable flow rate

Marina Slim S power filter5 star rating (5/5 by editor)Check on Amazon.com

Marina Slim power filters circulate four to five volumes of aquarium per hour that is optimal flow rate to keep the water clean but not stress the fish.


Adjustable flow control – image from Hagen

There is a regulator valve situated on top of the inlet pipe outside of the aquarium water for you to adjust the flow rate of the filter, so you don’t need to worry about the water flow is too strong for your fish.

Slim and compact design

Look at to the picture, you can see how slim this filter isLook at to the picture, you can see how slim this filter is

The Marina Slim filter is exactly that – slim but long: it stretches along the edge of the tank yet doesn’t stick out as far as other hang-on-back filters. In fact, it is only protrudes 5cm from the tank making it perfect for tight gaps. It’s length, on the other hand, means that despite being very slim, it can still hold a good volume of filter media.

Pump performance and noise


The pump that powers this filter is situated inside the tank, which on the face of it may seem like a bit of a shame given everything else is outside. Comparing this filter with other filters with an external pump, I would have to conclude that having the pump inside the tank helps tremendously to reduce vibrations.

It does still produce a bit of noise which I managed to eliminate using some neoprene tape where it comes into contact with the tank. Various users have reported through fishkeeping forums that the inlet pipe can make a rattle if you push it down too hard into its slots: this is true but avoidable if you place the inlet pipe in gently. The lid needs to be well positioned as well to keep noise at a minimum level.

While on the subject of the pump, it is fairly small as you would expect it to be and comes with a strainer and sponge cover. This is ideal for preventing fish being sucked up the inlet tube but also prevents large particles of waste clogging up the filter media inside.

Waterfall outlet


Most hang-on-back filters dispense water back into the aquarium using a waterfall and the Marina Slim filters also feature a waterfall. What is good about their waterfall is that it has bumps and lips along it which seem to kick the water out across the surface of the aquarium, a very good idea and useful for increasing gas exchange at the surface.

The waterfall isn’t covered like it is with many hang-on-back filters though I’m not overly sure that covering it is necessary. I personally have used the waterfall to grow mosses and it works very well.

High quality filter media



Model # of Bio-Carb # of Bio-Clear
Marina Slim S10 1 1
Marina Slim S15 2 1
Marina Slim S20 2 2

The Marina Slim power filters come with two types of filter cartridges: Bio-Clear and Bio-Carb. These cartridges are wool pads pre-packed with some types of filter media. While the Bio-Clear is filled with ceramitek (highly porous ceramic) and zeolite, the Bio-Carb contains ceramitek and activated carbon.

Ceramitek is highly porous ceramic for better biological filtration. Zeolite is a chemical filter agent that removes ammonia, dyes and drug residues from aquarium water. Activated carbon is an excellent chemical filter media for eliminating heavy metals, odors, impurities and discoloration that keeps the water clear and toxic-free.

If you don’t love to use the cartridges from the manufacturer, you can replace them with other media. The Marina Slim filters have enough room for accommodating some ceramic rings despite the fact that the internal space of these filters is only designed for use with filter cartridges. However, I recommend you to use the cartridges from the manufacturer if you use a Marina slim filter because they are very good and worth for money.

The filter cartridges need to be replaced every two or four weeks. It is important to NOT replace all cartridges at one time to not compromise the biological filtration capacity of the filter.

Pros and Cons of the Marina Slim S series

  • Slim and compact design for tight gaps,
  • Quality product keeps the water clean and clear,
  • Ceramitek excellently perform biological filtration,
  • Flow rate is adjustable,
  • Good waterfall for better gas exchange,
  • Automatic start-up,
  • Easy to set up and maintain,
  • Very quiet operation.
  • The filtration capacity is small and less versatile for using many types of filter media,
  • High operation cost for buying cartridges.


A good, efficient filter which takes up very little space outside of the tank and is perfect for tanks placed next to a wall where space is at a premium. The pump works well and with a few positioning adjustments can run very quietly after playing around with the position of the pipe and lid. There is oodles of space in the filter chamber for additional filter media and I would thoroughly recommend replacing all of the media it comes with in favor of plenty of biological media as it won’t clog up owing to the strainer and sponge.

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  1. Nice review! I think you missed one of the main advantages of having the return pump inside the tank – there is no priming needed to get the pump running again after a power outage as is the case with most other HoB filters.

    • Thanks for nice comment, DaveJ! Yes, I agree with you.