Your Eheim folks are thinking of the average aquarium with few if any plants, and oxygen/CO2 exchange is far more serious because there is no natural process there to deal with it. [/quote]
As I was also one of the one's suggesting the Eheim filter... I have to disagree there.
When I suggested this filter and was referring to my own set up's in the 55g's and larger I had severely heavily planted
tanks, planted to the extent that you HAD TO keep taking plants out every week and cutting them way back just to you could see your fish for a few days before the jungle had overgrown again.
However that said, in any of these tanks, by Eheim themselves they'd suggest something larger such as Model 2217, my 55g's were set (and are set up now as well) with the 2213 which does not produce that extreme water exchange like its larger Models. Also I always attach my water output on one short side, while the intake is on the opposite other short side of the tank and the output is angled downwards, maybe something like 40degree angle so it SLIGHTLY
moves the surface (barley visible to the eye) and it provides enough circulation under the water level for a proper exchange, but farrrr less then some filters that actually will make your plants swirl side to side.
And I'd want to say that the way my tanks were & are set up indeed had a natural process to deal with just like any of the heavily planted 20g's had for that matter.
IMO there's about 1 gazillion ways to set up a tank, double as much to maintain it and triple as much ideas & options from different people. In the end of all this, I personally strongly believe that its not ONE certain filter that works for a planted tank or ONE certain substrate or ONE fertilzier or only ONE type light ~ IMO a GOOD combination or rather a plant & fish PROPER combo out of all of these matters is what either makes a heavy planted tank work or die, its the balance of all these issues that the key IMO