It is not likely to bother your fish. Here is what Seachem says of it;
It is not mechanical filtration. As far as chemical filtration is concerned it is not meant that chemicals pull out material but that there is more than a physical removal of material. Purigen is part of the chemical filtration genera. It does not work as an ion exchange resin would typically work.There is some affinity for ammonia/nitrite/nitrate, but you are correct there is more of an affinity for organics. This is turn does help with water clarity, dissolved organic load, and ultimately ammonia/nitrite/nitrate.