Most ferts for aquariums get the needed nitrogen from nitrate. The spike use Urea. As the spike dissolve this turns to ammonia. However, ammonia in water immediately turns mostly to ammonium. Plants use ammonium and the nirifying bacteria prefer ammonia. There are nutrifying bacteria in the substrate and some plants will actually transport oxygen down to their roots an anoxic.anaerobic areas and then release it. The presence of this oxygen enables nitrifiers to colonize, This als results in area of denitrifying bacteria to form and they will handle and of the nitrate the pants do not use.
As long as one has several inches of substrate and pushed the piece of the spike down in that substrate, the Urea/ammonia/ammonium stays there. But when the spike is dug up the nitrogen gets into the water. This should not be enough to be a prolem for fish as the nitrifiers and water column feeding plants will use it. But so will algae and the result is an algae outbreak.
I have been using these spikes for over 20 years and never had any issues with fish and ammonia etc. I am currently in the process of greatly reducing the number of tanks i have. of 10 10 remaining, only 3 have no plants. Of the other 7 I still use jobes in the subtrate of in for plants in pots. I am still adding the spikes to 4. One 29 is bare bottom and holds only plants with none in substrate. The 150 is all anubais on wood and no substrate ferts needed. The 5.5 only has an inch which is not enough to use the spikes.
Here is a list of some of the fish which have lived in my tanks with Jobe's Spike in the substrate. Fish that spawned are maked with *:
(Excuse mixing Latin and common names but some common names help)
Clown loaches (oldest is over 20 years with me and 12 inches)
Ambastaia sidthimunki (dwarf chain loach) some about 20 years with me.
YoYo Loach (Botia almorhae)
Angels: Scalare* & Altum
Discus
Corys: Panda*, paleatus*, similis*, sterbai*, black schultzei, albino aeneus
Harlequin and espei rasboras
Danios: zebra*, margaritatus (CPD)
Sahyadria denisonii (redline barb)
Siames Algae Eater SAE (Crossocheilos langei)
Rainbows: Pseudomugil furcatus*, gertrudae*; Iriatherina werneri* (Threadfins)
Tetras: Cardinal, Rummynose, Purple emperor
Plecos: tank strain brisltnose reg.* albino* reg.* & long* fin. L450*
Montezuma Swordtails* several strains
White Cloud Mountain Minnow (anichthys albonubes)
Amano Shrimp (Caridina multidentata)
Neocaradina davidi- Blue Dream*
There are likely a few more species but its been 24 years in the hobby and I am old and that is the best my memory could do today