If you look at the actual papers pfk is reporting on for 2 of them links you will find the co2 levels they are using are 500, 750 and 900ppm, and its marine fish which are way more sensitive to water parameters. Compared to the max of 30ppm that is used in a planted tank.There is sufficient evidence that over-dosing plant additives is risky to fish. And with the EI you pretty much need diffused CO2, and this is without any question harmful to fish. I have discussed this with Tom a while back, and he is entitled to his view and without question when it comes to botany he does know. But he is not an ichthyologist, and he admitted to me that he promotes mainly plant tanks without fish or with few fish. He may or may not understand the issue with fish in these high-tech tanks, that really doesn't matter. I care more for the fish, and I am not prepared to make their existence more difficult just to achieve better plant growth. Especially when my tanks look like those below, I see no problem here, and the fish are better for this approach.
You may be interested in some article on the CO2 issue. Regardless of the specifics, the evidence is clear that CO2 does cause issues for fish.
Fish lose common sense as CO2 levels risewww.practicalfishkeeping.co.uk
The 3rd article is pure speculation and even says as much, including the fact that the issues he described could just as easily be put down to poor diet.
Again its just personal experience but in my tanks running co2 and ferts I have had even quite sensitive fish thriving for 4+ years. In that time I only had 2 losses. One was a Cory that got caught in a hair net I was using to hold down a plant (my own stupidity there I admit) and one was a flame tetra that had a deformity when I got it. Even that one lived a long time first.