I am running a pair of FX5s on a 300 gallon tank and they have proven to be solid workhorses. There are some elements of their design which I do not like, but there are other elements which are very useful...for example, the drainage port on the bottom of the canister. There is no question that they have the predisposition to exhibit quirky behavior and there are certain components which give them a cheesy look and feel compared to the larger Eheims...but they are about half the price and IMO a very good value for the money. They are designed to be a mechanical filter first and a biological filter second. However, on the appropriate tank, this can be a good thing. I have modified the media a bit by placing thin polyester pads in back of the foam pads and this modification has worked well to provide a finer level of filtration while also avoiding pad clogging. I have only been running them for about 9 months, so long-term durability is uncertain.