Blyxa japonica for 20 long?

It shouldn't grow more than about half a foot tall (eventually). Seems like you could use it midground, or even start with it foreground and eventually move it back.

It seems to depend on the blyxa j. I've seen some people grow theirs to like a foot plus, but most of the time it stays around 6 inches or so.

when it starts to grow tall it will have multiple plants attached to it. just seperate the plants and get rid of the bigger ones

I had it in my 20 long, used it as a background plant because of the slope of my substrate.

In my experience with this plant, it grows taller in lower light (6"+) and shorter in higher light. I can't tell you the exact point it switches, but it used to grow really tall before I got more lighting. You can see what I am talking about if you go to my journal here:

http://www.plantedtank.net/forums/775154-post54.html

Hope this helps.