FE CO2 is quite high grade, no issues with using it in fish tank. Obvious is has to be quite "clean" as you don't want people dying from CO2 impurities if they ever fight a fire.
It’s been awhile since I looked at the various grades and impurities ... or perhaps UK FE CO2 is held to a higher standard
Can’t say I “get” the reference to people dying from CO2 impurities whilst fire fighting - seems a non-sequitur
... rather like saying, chlorine levels in tap water are far from toxic to humans, so should be no issue for aquarium livestock