My Aquariums

It is a recommended soil by Walstad, and I believe it's an American soil as I couldn't find it for the life of me in this country.

http://www.bookmasters.com/marktplc/00388Shrimp.pdf

Walstad is talking here about her shrimpery, in the main book she doesn't recommend any particular brand of compost.  She also says "While any unfertilized potting soil will work, I have gotten good results using Miracle Gro’s Organic Choice ‘Potting Mix’. It contains well-composted organic matter, no chemical fertilizers (e.g., ammonium sulfate), and has a desirably low NPK ratio of 0.010, 0.05, 0.05." The levels of NPK in JI 3 are much higher than this. She also says "Once a terrestrial soil is submerged, inorganic chemical fertilizers will not only stimulate algae but cause major toxicity problems for plants and fish."

Maybe that's why many people using JI 3 seem to report initial problems with cyanobacteria, which are bacteria but physiologically behave like algae. Every summer there are warnings for people to keep away from some freshwater bodies which have blooms of "blue-green algae" (= cyanobacteria) which can be toxic to people (and dogs) as well as freshwater life.

I think you need a lake full rather than a tank full to harm a person. I know that dogs have died after swimming in pools with blooms. A local water park I know of is regularly closed for this reason, particularly after a long dry spell.

It's only a few cyanobacteria species that are toxic enough to be of significance to people, I very much doubt if there's anything to worry about, but one wouldn't want to put fish into a tank until any early bloom after setting up has been dealt with.