Fish health is not my area but...
- did you do anything different this year than
the previous winters?
Such as a fall clean out, the waterfall running during
the winter. That would probably be where to start looking.

One year I went into winter with a huge growth of anacharis
in the pond, so much that sometimes I could never see the fish. By
winters end it was all gone! I was sure surprised, and I assumed the
fish (a lot of koi at the time) chowed down on it sometime during that
period.

Also your fish have probably mulitplied in the time you've
had them. What may have worked for the orginal fish load
may not be working for the current fish load.

kathy :-)