Thanks for your responses!

I will assume that the substrate is the problem. I'll replace it in the half of the tank in which things won't grow and plant new plants.

Should I expect the substrate that I remove to be crowded with crypt roots? If it isn't, what will that mean?

There are plenty of nutrients in the water column, excepting carbon. If plants are able to make use of ferts in the water column, why won't the plants simply use the substrate to hold on to, and get their food from the water? The H. polysperma which I added a week ago should be able to do that if any plant can, yet they started dying on day 2 and are just about dead now.

I'll post the results of this in a few days.

Thanks again.

Bill