Let me preface by saying I have never used this product, nor even seen it, so my comments are not from direct experience.
The data on the website here Flex
causes me concern. All the "messing" it does with the natural processes can be detrimental to fish. Products that accelerate the breakdown of nitrogenous wastes create more ammonia (and possibly nitrite following) and this is not good. As for "buffering" the pH, this can be dangerous, depending upon the fish. Last, it seems to add minerals which means raising the GH (soft water fish would find this stressful) and adding TDS which is detrimental to most fish.
This also works as a flocculant [check here if you need to know what this means: Clarifying agent - Wikipedia, the free encyclopedia
] and these products are never, ever wise in a tank with fish.
As for maintaining healthy water, that we achieve naturally with live plants and regular partial water changes. Dumping some compound into the tank is never going to replace either; and after 18 years, if this stuff did a fraction of their claim, we would all know it.