The weather was just too good, and with no deep freezes expected in the next ten days, I "opened" my mothers koi pond for the season. This is an earlier date than I normally choose, but the warm winter allowed more off season decomp to take place, so the pond was in definite need.
My first act is to isolate and clean the filter and filter media very well. All the good stuff that kept the pond healthy last season (the dirt and muck and bacteria) will have mostly stagnated, frozen, and died, so I blast all my filter media until there is barely a speck of grime left. No doubt the dirt will be back soon.
I work hard to isolate my filter cleaning activities so the pond doesn't receive an influx of filth. Dumping a bunch of decomposing muck into a quite early season pond with cold fish is a recipy for disaster. This means closing off and then flushing out all my plumbing.
Ok, onto the easy part. For the pond I exchanged only a third or so of the water, removed some of the sunken leaves, and trimmed off dead pond plant vegetation. All filled up again, I plug in the pump and clean water starts rushing down the waterfall for the first time this season.
I don't recommend a 50-100% super clean that some people do in the early spring. It can be very stressful for the fish and in cooler temperatures, they just can't deal with it. Remember, if they get sick, it's very hard to treat in cold water. I'll follow up this pond "opening" with another partial change soon. Might even do a 50-75% exchange next, but by then the fish will be ready for it.
Sam