The heading on an MSDS is "Dangerous Chemicals" but that does not mean that the product cannot be used safely. Sodium Hydroxide can be very safe, there is a lot of it in soap, for example, and if you eat grits you are eating sodium hydroxide in the hominy. For that matter, a sodium hydroxide solution is used to wash fruits and vegetables, in chocolate and caramel production, and is in every soft drink. Tartaric acid is present in wine, in fairly high concentrations. Formaldehyde is probably the most dangerous, but a solution of 2.5% or less is not very dangerous, and can be very safe if used properly. It is a mild anti-microbial agent. It is a principal component in a lot of ich/other parasite medicines, though in those cases it is usually around 20-35% concentrations. I guess my point here is that many, many, many things have "Dangerous Chemicals" in them, but that doesn't make the product itself dangerous, so there is no need to freak out about it. I am 100% sure that those chemicals are added because they perfrom the job they are supposed to perform. If all they did was kill fish, I am sure that by now the fishkeeping community would know about it and Tetra wouldn't still be making this product.
Now, about its use, I do find it highly suspect. There was a great thread about 2 years ago now where a forum member kept two tanks identically, one with the regular water changes and one following the intructions on the EasyBalance bottle. Bith contained a populaiton of guppies. In the water changed tank, the guppies spawned normally and were their normal guppy selves. In the EasyBalance tank, the guppies were healthy, none died or anything, but they didn't spawn. And I think as we all know, guppies that aren't spawning have something wrong with them. It seemed like it was nothing overt, but the guppies were obviously not 100%.
That thread and many others talking about EasyBalance can be found using the site's search function.