In my vivariam i use a false bottom and hydroton to ensure proper drainage of my soil. This method works fine over the summer, however this is not the case over the winter when i have to use my under the tank heater pad. When the heater is on it causes the water to evaporate into my soil. This is killing a few of my plants because the soil is too saturated. Any ideas?

What are the room temps? I'm wondering if the heater is needed?

Instead of a heating pad..check ambient room temperature..and heat room as necessary..

If you feel that the room can not be heated and must use the Under the tank heating pad..try making a pool in the corner of the tank or placing a Pipe in the Corner..the evaporation should escape that way

Where are you lacated?

Just like Glenn said ambient room temp is the best. I know when it is too cool for me is too cool for my frogs.
Just a thought.

There is a new product that Casper and I are experimenting with it is called Exo Terra Tree fern tropical substrate. Looks like it would hold up better when mixed with other substrate along with better drainage. It is not the same as the tree fern fiber found.
Much finer.

Will post some updates.