+1 on all of Byrons advice about stocking modifications, filtration, and fertiliser. As always.
The mystery plant- has the plant put on any new growth? It looks like the emersed growth of Proseprinaca palustris...
Here's a pic-
The middle stem is the emrsed growth- the leaf on the left is the submerged growth. In my experience it's an extremely hardy plant. The emersed leaves don't fall off once submerged- they stay on for a long time. (Until smothered by algae, which explains a bit.)
It could also be hygrophila (a strange variety, or as Byron mentioned, low-light growth. Of course, with flourescent lighting, it shouldn't be dark enough for leaves to look that way. Hygrophilas are durable IMO however, so if you like stem plants, Hygro Difformis is a good one.)
It could even be Mexican Oak-leaf (Shinnersia rivularis), but again, the leaves aren't quite right.
The stems look thick, so that's another reason to think it's the emersed growth of Mermaid's Weed (P. Palustris).
I'm a fan of dwarf sag, echinodorus bleheri "compact", aponogetons (Don't live more than 6 months or so, but are gorgeous) , Java fern, Java moss, and crypt wendtii. (crypts are finicky if you're moving things around though.) along with the ones mentioned above.