Two seventy degree days in a row here caused me to poke around my pond.
All my Koi are active and lived. I had covered 98% of my pond with a
pool cover (basically bubblewrap) and used a heater to keep it from
freezing over with 2 airstones.

I tested the water temp last night and it was over 60 (near the top,
depth is 30"). I had a heater in there this winter which I just shut
off yesterday. I fed the Koi lightly, but am wondering if it is too
early to put in the filter. We are looking at lows in the 30s in the
next few weeks and during the day can be 40-70. I am thinking I can
put in the filter and if the water drops below 50-55, the bacteria will
die, but no harm done. Any thoughts?