So I finally found the exact Alternanthera species that lines my river wharf. Apparently, it was an old hobby cultivar named "Kleines Papagaienblatt".
Link to description. Apparently, it's a medium difficulty plant for which CO2 is recommended, so, if true, growing it in a non-CO2 Walstad is a pretty cool success. It has been doing great in my small 25 liter tank.
That said, here is a pic of the three-day old tank. Some clarifications. I glued the wood together and placed a rock on top of it until it soaks through again, I had kept it all in a pond for weeks but as I moved it and stored it in the flat it dried up faster than expected. When I set it up and filled the tank, I removed the rock tentatively and it stayed down... For five minutes. Mess No. 1. Stuck it in as best I could again, and placed the rock once more. It does not look as seamless as it did before, but well. Once it soaks I will rearrange so that it looks better. For now, I'm playing it safe.
The second mess was that a good percentage of the plants I had in storage for this suffered enormously from having spent a week in bags... So I had to go out and stick some Frogbit and other unplanned inhabitants, as a good plant mass is essential if I want the firing up to go smoothly. In three days the water cleared up, the plants perked up and things seem to be moving in the right direction. Right now, I have a bag of new plants I collected at 23:30 pm on a rainy night... People coming by the river must have thought I was concealing murder evidence or something. It's a pity I had to leave my parent's house and come downtown, with the rain and some recent mild floods the temporary pools were blooming with plant growth. Here go some pictures.
Lilaeopsis, Eleocharis, Alternantheras, and some of the small ones that worked well on the other tank. I'll replant, rearrange and get back with more ASAp.