Nexx external filter review


Editor's Picks
Practical Fishkeeping Readers' Poll 2023
Fishkeeping News Post
Readers' Poll 2023
07 August 2023
Fishkeeping News Post
Countdown for Finest Fest 2023
20 April 2023
Fishkeeping News Post
Pacific Garbage Patch becomes its own ecosystem
20 April 2023
Fishkeeping News Post
Newly described snails may already be extinct
20 April 2023

Jeremy Gay puts the new Nexx filter from Mars Fishcare through its paces...

When I heard that Mars Fishcare, owner of Rena, Aquarian and API had a new external filter my eyes lit up. However, after setting one up and using it on my own tanks, this won’t be replacing any of my all-time favourites.

Give me a Fluval, Eheim, JBL, Tetra or even a Superfish external and I’m fine. So invent a new external filter quite different from the norm and it has to be better than anything else. After trying the new Nexx I’ll be sticking to what I know and love — at least for the time being.

Hagen lifted the bar with its Fluval G series external filters. These can be maintained while still running and even be opened for mechanical and chemical media to be replaced in seconds.

This must have sent many rivals back to the drawing board and Mars has responded with something truly different, incorporating both Rena and API technology  — the new Nexx.

It’s good…

It is self priming: Despite most filters having easy prime buttons or plungers, some fishkeepers don’t get it. Filling external filter canisters has to be idiot-proof and I’m not talking a jug and funnel here as they get a big zero for cleverness.

Nexx takes all worries about priming out of the equation, doing so by placing the pump in the main tank above the filter, instead of mounting a head unit directly on top of the filter in the cabinet. Put the pump in the water, connect pump to filter via supplied hosing, switch on and the filter fills with water. You can’t get it wrong.

It’s modular: This can grow as your fish, your tanks and your needs grow. Buy a Nexx and it’s complete and ready for aquariums up to 200 l/44 gal. Add more fish, like Malawi cichlids, messy fish like goldfish or upgrade to a larger tank and, instead of buying a whole new external filter, simply buy an extra Nexx filter module and bolt it on to your existing one.

Same pump, same single power lead, just more filtration.

Seasoned UK fishkeepers may remember the Eheim flow-through modules for sale via mail order in the 1990s. I’m a huge fan of extra filtration and being able to double, or even treble it in the case of the Nexx is a very good thing. One extra module makes it good to filter 400 l/88 gal and two extra modules is good for 600 l/132 gal, or roughly a 1.8m/6’ tank — says the manufacturer.

If I could afford to I would have at least one extra module, even on a small tank, to get the best filtration possible.

Maintainable while still running: Each filter module has two cylindrical chambers. With the filter running normally, turn the yellow handle on the top, cutting off the water flow, lift the filter off its mounting in the cabinet and walk off either to bin or the sink.

Mechanical and chemical media are taken care of by carbon-impregnated pads stretching over a plastic frame. These are of Biochem-Zorb — one of my all-time favourites.

This means that water may not bypass the mechanical and chemical media, creating very clean and clear water. The two can be thrown away together on a two-monthly basis and the biological media can be left completely separate and biologically intact.

There are two in each filter module, so with three modules you have six working mechanical and chemical filter modules, known as the crystal pouches, and others are available to specifically deal with, for example, high nitrates or algae reduction,  

Once you know how to do it we’re talking minutes tops and Mars quotes four minutes as maximum to remove and swap media.


Not everyone likes a pump in the main tank and planted tank aquascapers I know won’t take to it. The in-tank pump is an eyesore, you can’t swap it for near invisible glassware and the strainer is just the small inlets on the pump itself.

There other arguments against pumps in water, like heat transfer, although I’m sure that would be minimal on a pump such as this.

We also challenged Mars on filter flow production. With one module it produces 500 lph but increases to 580lph with an extra one and 630 lph with two extra. Power consumption also goes up from 11 to11.8w on the highest flow.

However, there isn’t a microchip-controlling flow and it’s more like physical restriction from pumping water through one module but less restriction from pumping through two or three of them.

For a 600 l/132 gal tank a 630 lph flow will seem woefully inadequate. I’d want at least two 1,000 lph external filters on a 600 l/ 132 gal tank, or probably a Fluval FX5 at 2,300 lph and enough guts to filter a tin of baked beans.

Mars responded: "The efficacy of API® media makes it possible to filter water almost in one pass, excluding crystal testing before/after.

"We must avoid confusion between water circulation and water filtration. Where efficient water circulation requires high flow rate, the quality of water filtration may require a lower rate. Slow filtration is also known for best efficacy.

"We must avoid confusion between pump flow rate and filter flow rate. Packaging claims are sometimes confusing and consumers may compare one with the other, although they are different. We must also make the difference between the flow rate of the empty filter and one of a filter loaded with media.

"The pump is set to its maximum flow rate on the assembly line, and not adjustable by the consumer. Most people use their filter at maximum flow rate and there’s a risk of negative perception if the end-user does not notice minimum flow rate."

But then there’s the amount of media in the filter itself. The mechanical and chemical filtration is fine, but in relation to the size of filter and what we normally see provided in competitor filters there are only a tiny amount of Bio-Chem Stars biological media, and it seems wasted space.

Mars publishes more scientific papers than anyone else in the pet trade and I’m sure that Bio-Chem Stars are highly efficient and have a huge surface area in which bacteria can colonise. This Mars can prove, but I would have liked to have seen more space given over to them.

I also don’t like being tied into bespoke mechanical and chemical media replacements, preferring being able to use someone else’s sponge or filter wool and carbon.

I won’t be able to bodge my own money-saving media replacement pouches. Pre-filtration foam pads are available, but if they could aid filtration in certain situations why aren’t they actually supplied with the filter?


I so wanted this filter to be brilliant, so will set it up again and use it for an even longer period. However, for all its fresh thinking, I can’t put it top of my list.

Ease of disconnection and maintenance is very good, but I have reservations over the pump in the main tank, low flow on big tanks and the small amount of supplied biological media in relation to filter volume.

Prices: Nexx filter (RRP) £104.99; Nexx extension (RRP) £74.99; Nexx Crystal (two pack) £12.99; Nexx crystal (four pack) £22.99; Nexx Aqua Detox (two pack) £12.99; Nexx Prevent Algae (two pack) £12.99; Nexx Pre filtration foam (two pack) £11.99.

Check out Jeremy's video showing how to set up the new Nexx filter.

Why not take out a subscription to Practical Fishkeeping magazine? See our latest subscription offer.