Can't remember ever having this phenomenon before in all the years I have kept koi.
Result was I had to treat my pond over Christmas Holiday as I was due to go away in early January on holiday for a couple of weeks and needed everything running smoothly for my neighbor to keep an eye on.
Good thing was everything did run smoothly filterwise as it turned out.
I returned home to UK on Thur last from 20+ degrees and stepped off the plane to snow flurries, yuck.
To date there are flies still hatching from filter a good sign I think as now the filter is enclosed with 25mm polycarbonate water is noticeably warmer.
Problem with flies hatching continually is there is a marked increase in spider population inside housing.
Most annoying is they spin their webs across door frame so when you open door and walk in the horrible clingy filaments are dragged across your face and hair, ugh
.I have spent hours at times removing webs for them all to be back next morning.
I read somewere that spiders don't like horse chestnuts so after visit in Autumn to local park collecting conkers I duly placed lots of conkers around filter house.
Either I have wrong sort of spiders or the spiders I have never read same article, anyway, the conkers never worked.
I would now rate conker averse spiders along with garlic for witches, silver bullets for vampires and bic razors for werewolves, don't waste your time!