I would have to say that the water temp is to warm for elodea. It is
a plant that does best in colder water, around the mid 60s. Even
though it is widely sold by many LFS, it naturally grows in slow
moving water that is cool. Someone else may have more insight on the
problem. The above is from my observations.

Thanks,

Now that's an interesting thought, Leigh mentioned it as well.
One variable here that has changed is the *location* of the heater in the
tank. I moved it over to the Egeria's side of the tank a while ago. It only
comes on at night, but perhaps the *local* temerature in the tank near the
plants in question gets too hot. The heater may have been creating a hot
spot in that area of the tank, as circulation was not great. The powerhead
injecting the CO2 has added a lot more circulation but it may be too late to
save this group of plants.

Lee