Hello, got a question for you pond experts please...

I live west of Houston, TX and we are in a two year drought.  Most ponds in my area are dry.  I dug a well to keep my pond full.(about an acre and a half)  Before I started filling the pond with the well I had nice hydrilla below the surface in a lot of my pond.  I have tilapia so it was under control and I really liked it.  I turned on my well and filled my pond back to the full level. Ran the well for about 5 weeks to get it full.   Probably added 25% to get it back to full.  For some reason now all my hydrilla is dying and decaying.  Can something in the well water be causing this?   Lots of scum is rising to the top of the pond.  I think it is the decomposing hydrilla.  I haven't noticed any fish kill but I sure hate to lose my hydrilla.  

Any ideas or have you heard of anything like this before?

Thanks