I just set up a 20 gal walstad tank with 3/4 inch of miracle gro that I soaked beforehand. It has 1.25 inch of seachem flourite substrate cap. It’s only been a week and I have planted a LOT of rotala, ludwig, and a few vals and wisteria. I noticed the tops of all the plants look very healthy and seem to be growing, but the stems I checked when pulled up from the substrate, are rotting at the base. I have guppies (doing fine for now), and trumpet snails for the soil but I realized that they can’t even dig beneath the flourite substrate I chose. If I poke anywhere lots of bubbles come up from the substrate. Worried about anaerobic conditions. Is there anything I can do to prevent or fix it?