The other type of system you are referring to (I think) is referred to under several different names and they are all related and differ slightly on the specific set-up used. Sop let me throw some names out that are all related and then I'll explain why I like a canister/Sump combined method:
Sump, Wet/Dry, Overflow, Refugium. These are all related to the method of using an overflow box (or overflow plumbing) and a seperate tank under the main tank referred to as a sump. These Sump/Overflow systems can be set-up in a variety of ways (Bio Balls, Carbon, Skimmers(for SW
), etc.). This approach mainly came from the SW
systems of today and Wet/dry systems (Which are Sumps which Basically contain adrip plate and bio-balls for bio filtering).
I part of this approach is very applicable for FW
as well. I posted on another site this basic approach for FW
tanks: Use an overflow system to deliver water to a Sump under the main tank (this is only a delivery system to get the water supply for your main filter - I'll explain why this is good in a moment). Then pump that water out of the sump into a Canister system loaded with your favorite media (and include a bio media like Seachem Matrix). Then the return of the canister goes back up to your main tank.
The advantage of this system is that it takes water from the surface of your tank - this is where most of the DOC
's reside. Have you ever noticed slime on the surface of FW
tanks? This is the protiens and DOCs that we want to feed to our bio-system. Since this is the water that is delivered to the sump and which then flows into the canister - voila - you are bio-filtering where it is needed most. This also frees up the surface area of the tank for increased O2
I'm not sure why the overflow approach seems to only be used in SW
systems - it is clearly very applicable to FW
systems as well. You will never see that ugly film on the top of your water and your bio system will function at it's peak.
Also since you now have a sump - you can put your heater in there as well, and you can use the sump to dose any additives you might want to use.
I am a big adovacte for this type of approach to FW
systems and if you have any questions about my explanation please let me know.