Problem as I see it ,Is the miracle grow potting mix,and some other soil,substrates ,can release ammonia for a few week's from organic content of the soil.
After a few week's,,the bacteria will have had time to establish itself in and on the substrate, but to add more than one or two fish to small bowl's,or tank's less than five gal,will require substantial investment in enough plant's to keep ammonia from poisoning the fish.(one or three plant's may not cut it).
Believe (my view) this is why Walstad does not place many fish in these small affair's, and it would in my view,,be wise to do likewise.
Plant's can easily use up ammonia from fish,and perhap's that which may leech from the soil,but it takes a fair amount of plant's from the outset.
Do also keep in mind that plant's often need time to establish themselves or adapt, to submerged condition's after being grown emmersed (root's below water ,leaves above) and during this time ,they may not be as helpful at removing,assimilating ammonia produced by fish and or organic content of substrate.
Every body is alway's anxious to place fish in their tank's ,(me too) but letting the tank and plant's get esablished,and mature a bit,,will make condition's more livable for fishes,invert's. IMHO.