IMHO, those are overrated. They seem to make the entire overall frame of the system weak, though I've never used one so I can't say if perhaps it just seems that way in the store.
They are also seriously overpriced. They were running $60 for one at the local petstores for a 20L, which was running $35 at the same store for the regular 20L. And for what purpose? Either way you have to buy a filter of equal size.
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1.0.0 Thoroughbred
1.0.0 Mali Uromastyx
0.1.0 Bearded Dragon
0.1.0 Crested Gecko
1.2.0 German Shepherds
1.2.0 Cats
0.1.1 Birds (Cockatiel and Conure)
1.1.0 Rabbits
0.0.2 Yellow bellied sliders
3 Fish tanks (SW and FW)