A couple questions:
How deep is your tank?
What is the wattage of the MH's?
At what depth is the toadstool?
What lighting did you have before, or is the toadstool new?
For what it is worth, I have several softies (toadstool, finger leather, shrooms, polyps etc) under a 150w MH. They are all thriving, but are lower in the tank and some are protected by shade from the rockwork. MH in and of themselves may not be the issue, but rather placement relative to the light.
Also, if you have just upgraded your lighting, you may want to transition them by using layers of window screen below the light, removing a layer every few days allowing the system to acclimate to a more intense lighting.
Now that being said... if you just put 2 400w MH bulbs over your Nano, you may have some other issues!
