I have a few tanks and out of all of them my little baby 2' tank (I think about 15 buk gallons) has a Eheim 2010 internal sponge filter. It turns over 110 uk gallons an hour at its maximum rate and the water clarity is second to none. The fish don't swim in the tank they.....FLY. Absolutely crystal clear. Mind you there is a little hurricane in the tank and you can see the fish swim against it when you watch them.
The filter I have does say 110 uk g's an hour but once you get the media in the filter it is not as much as the manufacturors say.
In my other tank in the dining room it holds about 75 gallons of water and I have a 1000 L/ph internal along with 2x2000 L/ph external cannisters filters called Sun Sun.
I myself believe that the sponge filter carries a lot of BB's that is great for Bio. I once had a tank that was WOEFULLY....... overstocked and I never had an ammonia reading at all the BB's in the sponge filter dealt with it perfectly, it was just the Nitrates that I had to deal with every second day tops until I up graded the tank.
So sponge filters are woefully unappreciated I think also
......