External Filter

Even the FX4 might be overkill on a 200 litre tank. That one is rated for tanks up to 1000 litres with a maximum flow of 2650 litres/hour - over 10 times the volume of a 200 litre tank per hour.
If you can turn the flow rate down, it might be OK. For 200 litres you need a maximum of 1000 litres per hour; less if you have fish from slow moving water.

I would look at something a bit smaller/less turnover.

I also think the fx6 is a hit much for a 200 litre tank, though regarding the fx4...

Many would suggest you need to shoot for at least x10 turnover in a planted tank to get nutrients co2  moving around efficiently.to aid plant growth. I have roughly x5 turnover as Sue has suggested as a maximum however would not consider this too much. This is because the filter is unlikely to ever produce this much flow in reality... unclean hoses, media, height difference between the filter and tank and other factors, bring the theoretical flow rate down significantly. My problem is normally the intake getting blocked with debris, something I have to keep on top of each week. Many filter manufacturers test flow rates with no media in the filter!

Sue is right that it depends on the type of fish being kept which is not something an aquascaper would necessarily place as much emphasis on. My five times theoretical turnover would not cause any issue even for fancy finned fish I do not personally believe as real life flow rate is likely to be far less.

Other factors to consider that Dave has highlighted are noise and power consumption. Noise certainly is a big factor for me as the tank is in our sitting room... it's not something that's easy to find out though and normally forums are the only way to compare noise from different filters, it's not something manufacturers necessarily report.

Further considerations include whether you would like an integrated heater. This is something I always want in a filter to hide this extra equipment from the tank for aesthetic reasons. Ease of maintenance... I believe the fx series have a drain port. The Oase Biomaster series have a quick release pre filter so regular filter maintenance is a doddle. Hoses... many Fluval filter can only use Fluval specific hoses and therefore inlets and outlets. I have glass lilly pipe inlets and outlets again for aesthetics and these would not be compatible with these filters.

Sorry... lots to think about there!! Not sure if I've helped as all... If you wanted a suggestion, the filter I'd recommend is the Oase BioMaster Thermo 350.

Thanks for the replies, i did think the flow might be to much for my community set and i know people are having problems using such filters on medium size set ups. I like the look of the Oase BioMaster Thermo 350 filter and the combined heater is a great idea but it seems a little expensive at £200. I'm now looking at the Fluval 306 which seems a bargain at just over £100 with free next day delivery, and with an increased flow of 1150. Is anyone using such a filter or has any experience of the Fluval 306 ?. I shall do a little more research on the Fluval before I buy but It's ticking all the boxes for me at the moment :-)

I bought a Roma 240 tank/cabinet, and that comes with the 306 filter.
Unfortunately I haven't unpacked that one yet, so can't really comment from personal experience. Sorry.

Best of luck with your new external filter, I hope that it's exactly what you need. 

Matt replied while I was typing, so just to expand on what he said -


A couple of weeks isn't long enough. If you still have the same fish load, the bacteria will stay where they are, there is nothing to make them migrate to the new filter.

The best way is to take all the media out of the current filter and put it in the new one. Then add new media to fill up the spaces. Wait a month, then remove part of the old media and replace with new; repeat after another month, and so on till all the media is the new media that comes with the filter.

Hagen doesn't give the 307 manual on its website but other sites give the media as biofoam max, biofoam, biofoam+, carbon and 'quick & clear'. You don't actually need the last two. Keep them in the cupboard and put the old media where they are supposed to go.

You could always leave the old media there permanently; or when you remove a bit, replace it with sponge, any make cut to fit.

The U4 has sponges and biomax (and carbon pads). The simplest would be to lay the sponges flat in a basket and fill round them with biomax. You can cut the sponges to make them fit.