This is a very cool experiment. Makes me want to try one of my own in the huge ceramic pot I have on the front porch.
I think this is your mystery vine:
The grasses look like common lawn grasses to me too, but I didn't think they would grow in water that deep. Is it possible they are some sort of dwarf rush? You could always pull a little and take it into the Master Gardener's office at your local university. They usually do IDs on plants for nothing. Part of their certification process I think.
I have some Peruvian Daffodils growing in my yard they ID'd for me last year. Of course they'll never bloom, not hot enough around here... (and that wasn't at ALL off topic-heehee.)
BTW, I think the pond link on that nano page is currently broken (homestead page not found). Looks really awesome though so far. Be interesting to see if the plants try to emerge. Tired...rambling...nuf said...