Seachem Prime, for me....8+ years, never had an issue with it
You've just hit on the crux of yet another problem in this hobby, something that is expressed very well by Nathan Hill in the blue citation in my signature.
I often think that when Seachem or API or whomever says "x" is "safe" at five times the dose or whatever, what they are really saying is that it won't cause the fish to turn belly up. But that does not mean it is "safe," or will not somehow affect fish.
For several years now, following lessons learned due to dying fish and discussions with marine biologists, I have believed and promoted the concept that absolutely nothing should go into the tank water (when fish are present) unless it is absolutely necessary for the good of the fish. Taking Prime, it messes with ammonia, nitrite and nitrate, and Seachem either cannot or will not say how it does this, aside from "binding." If you do not have ammonia, nitrite or nitrate in the source water (note, this is source water, not tank water) then there is no need whatsoever to be using this additive. There are safer alternatives.
It can take years for problems to manifest themselves in fish. The study on the effects of hard water on cardinal tetras is a good example. The study focused on groups of this species that were kept in water with varying degrees of hardness, from very soft up to fairly hard. The fish in the harder water did not live as long, and upon necropsy they found calcium blockage of the kidneys due to the hard water over the years to have been the cause of death. There was nothing noticeable to the individuals carrying out this study, until the fish died.
This is why we should never assume the fish do not have an issue with this or that just because they continue to live, to eat, even spawn. Fish like all animals have a strong will to survive and procreate their species, but the fact they may do this does not mean they are in good health.
