If the bottom leaves are falling off, it could be because they are older. It could also be because they are getting shaded out by leaves that are higher up and then dying.
Finally, the another possibility is that the plant is adjusting to new parameters in your aquarium and is dying back a bit. This is completely normal.
+1 to the adjustment statement. Specifically with repens they take a couple weeks to adjust. Mine wilted for 2 weeks while it was getting adjusted and then EXPLODED once it got used to my dirt tank. Just be patient and, like me, you'll have clippings you can sell or use for another tank.