| here's what I'd suggest based on the following experiences
In my progression as a koi keeper, I soon focused on controling temperatures
for my outside pond. The water would get down in the thirties and was really hard on the residents. We also get episodes of snow so I built elaborate plastic and wood framing. which didn't vent properly was ugly and not always snow load proof. I laid 4 x 8 foot 2 inch thick styrofoam sheets on the pond and that kinda worked. I bought a swimming pool cover (bubble wrap ) to fit edge to edge and that turned out to be a nightmare. How my first asagi from hosokai and the first babies from diamond mask (sakai hiroshima) got out on top of it and died I'll never know.
So I gave up, built a fish house with an inside pond and heater and brought everything inside.
here's my suggestion tho in your case: get some kind of pellets you could float for insulation. It would still allow gas exchange and if a fish jumped out it would crash thru back to the pond! I would anticipate it would be a chore of clean up in the spring. |