Depending on where you are, it might not be necessary to prepare your pond for the winter. If you water temperature stays above freezing during the winter, you probably aren't even thinking of putting a heater in your pond. But for those of us who are so lucky to live in place where it can reach below freezing temperatures during winter (and sometimes even in spring), having a heater for your Koi seems like a natural thought, right? Well, even though your intentions are good, having a heater in your pond can actually harm your Koi. This may sound counter-intuitive to you (and it was to me at first), but trust me, it's true.
At Professional Ponds, we are trained to not ever recommend putting a heater into a pond system, no matter how cold it can get. The simple reason is, that like all electric things, these heaters can fail (especially when they are outside) and if you or anyone else isn't there to fix it, or you simply don't realize it in time, your Koi fish will come out of the situation permanently hurt in the very best case but the reality is is your Koi will most likely die.
And no, it isn't because they will freeze to death. Like we've mentioned in previous posts, Koi fish can naturally adjust to the temperature of their surroundings but it takes time. If you allow your Koi to gradually change with the season it causes less stress and it is generally better for them overall. On the other hand though, if the heater in your pond dies suddenly, your Koi won't have enough time to catch up to the dropping water temperature and they will enter a state of shock, or stress and that is what harms them.
Instead of a heater, the team and I recommend a De-icer or just something that will prevent ice from covering over the entire pond. That way the harsh and toxic chemicals that are naturally released in your pond can escape.
Last season, we had a handful of customers come to the greenhouse wanting to replace fish who had died the Winter/Spring before and, just like anyone I talk to who has lost their Koi, I am broken hearted over their lose. Especially when it can easily be avoided! Hopefully this has been eye-opening for you, and you can take this knowledge and share it among friends to prevent as many Koi deaths this season as possible!