Cool idea to just add little bit of dirt to fish tanks to innoculate with. Wouldn't you wanna tea bag it at least?
Worry about parasites in the soil, especially with lungs aquatic animals?
My turtle aquarium has been crystal clear for years, and hasn't broke the cycle since I moved it last time. I'm careful about rinsing out all the filter medias with the same water, as to not wash away or dry out the colony. Its been awhile, but I kid you guys not, the fastest I've ever rooted cuttings was placing them in the basking area above the tank, down through the platform drainage holes, so the end of the cutting is hovering right over the spray bar coming from the pump. Right over the water level, where it splashes with lots of DO bubbles. Just like a DWC cloner, but organic/aquaponic style. My turtle would keep scratching at the ends of the cuttings (I though about adding a protective cage, and keep using the tank as my main propagation center at the time, lol) and trying to eat the root nubs, and I would just let him. He would swim past them to scratch his shell and little turtle butt. You would think its bad for them, but it actually accelerates rooting by like 5 days for some reason! Like, 3 days of being shredded at and you have roots! They were exposed only to a halogen, and also a concentrated UVB light btw, which is supposedly the worst lighting possible, so they say..