Several of my plants(lysimachia, HM, and cardamine lyrata) will not root in the substrate. When I first bought these stems, I planted them about 1/2-1 inch deep in the substrate. After a few days or a week, the part of the plant that is under the substrate begins to die off, and the bottom half of the plant weakens and turns brown or yellow. The tops of the plants grow very well and are fine when left to float above the substrate. These plants also grow plenty of roots at the leaf nodes. As soon as they are planted, however, it is only a matter of time before the bottom decays and the plant floats away. None of these plants will grow roots into the substrate.
I am not sure why I am having such a difficult time getting these specific plants to grow. None of the other species in the tank seem to have any issues with taking root. I've tried switching substrates, increasing the light, moving the plants, planting them in different ways... nothing seems to be working and it is getting frustrating. Does anyone have any ideas as to what the problem might be? Any help would be much appreciated!
Some tank info...
Turbo CO2 Biosystem by Red Sea
96W coralife fluorescent light
Penn Plax Cascade Canister filter
Substrate is Eco-Complete
dwarf baby tears
This is my tank...
the tops of the lysimachia look happy, right? the HM tops(not pictured) are similarly healthy.
the bottoms of the HM. it's not as easy to tell in the photo, but they are definitely brown and rotting.