If you can post another picture when they straighten out a little bit, that would be great. I agree that they look like ludwigia repens, but it's hard to tell for sure when they're contorted.
You want the lower leaves and stem to be unobstructed by the neighbors, especially in lower light levels. This type of ludwigia really likes to melt at the stem in moderate light levels.
Depends on what the rest of your setup is. You'll want "high enough".
I'd lean more toward ludwigia ovalis as far as the ID goes.