The only thing I dislike about the filter on my spec is that the water in the pump compartment stays pretty stagnant. So when I don't have my breeder box hooked up I run an air pump in there to get the water moving. Also I plugged the small hole near the bottom. If I had to guess they made that as a fail-safe in case the water level gets so low it doesn't reach the overflow part.
I stuffed a heater into my pump compartment, and then I cut a ~1/4" hole in the outflow line to keep the water level higher in the pump compartment for the heater. There is definitely no stagnation in that chamber on mine, even when I turn the flow all the way down (which is how I will likely keep it).