External filters come in different sizes and "fancyness". I quite like TetraTec EX series, I have two EX1200 and one EX700. I don't know what the list price is, but the EX1200 is around £100 and the EX700 is about £80, if memory serves. There are usually online sales of these things for a bit less, but then you have the trouble that if something isn't working for whatever reason, your LFS may not be quite as willing to help out...

The Mercedes of external filters is Eheim. Their Pro range is REALLY good (better than the TetraTec EX series), but at the same time, they are more expensive - around £200 pounds for the ProII 2028, which is equivalent to the EX1200. Eheim's Classic series, which I have a few of are less expensive (about the same as the TetraTec equivalent size), but do not have baskets for the media, which is not exactly helpful on those few times each year the filter actually needs cleaning.

I'd recommend going for the next size up filter (no matter what type), so if you have a 200 liter tank, and there is one filter for 100-200 liter, and one for 200-400 liter, go for the 200-400 liter size. That way, you have spare capacity, and it requires even less cleaning.

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Mats