With my sump I had two overflow pipes going to the sump.
One had a valve to reduce it until it had a full siphon. This stopped air being sucked in, reducing added oxygen.
And the second covered the extra overflow of water if the level got too high. eg. 1st pipe blocks up or siphon doesn't take full load.
My sump worked perfectly fine for me with my planted setup. I guess you have to set it up properly for it to work.
As said above, you need to reduce any extra aeration to the water or Co2 levels need to be increased. And the fish don't like that so much if the Co2 gets too high.