This has been an interesting spring for me regarding my little goldfish ponds and recent experiment with small koi.
The koi were thriving, I had hooked up a solar system just for their water filter in a 30 gallon aquarium, 9 small koi--45$.
They doubled in size over the winter and this spring I was debating on where to put them next, the sun wasn't shining , the filter wasn't running,as much, and I was away every other week, and the water was noticeably cloudy- I planned to do a water replacement the next day, weather heating inside, algae bloom, fish went from happy, hungry, to dead overnight.
---good lesson in sensitivity to oxygen levels- I think.
Now the goldfish outside, thriving in all sorts of conditions, wanted to give them a clean pond, switched them to a different prefab plastic pond filled with rainwater--no problem, recently decided to upgrade that to a bigger prefab I had just repaired, and hooked up the supply from my big pond, filled it with "nicer" cleaner water, made the transfer, and they immediately wanted out, trying to jump out, but pond was only half full, so I just puzzled a bit as I admired the nice new water and complimented my resourcefulness using the pond water.
three fish (out of 11) died overnight, and the rest were sluggish, so I got right on switching the supply to my rainwater system and adding that water, while I hooked up the carbon filter (the koi didn't need it anymore) and desperately tried to fix things.
Seems like it may have worked, remaining fish survived the night and were eating this morning.
Oh, I didn't mention the cat--neighbor's cat was showing lots of interest in them that first day, and short white hairs floating all over the surface of the pond that next morning. Did the cat have something to do with it?
Also, a detail about the earth pond-- when I dug it it filled partly from underneath with a blue green water, and that tint appears periodically when runoff and sediment and algae are at a minimum. Does the water have some toxic mineral in it?
Since a pond is the whole world to a fish, it has life supporting limits, and I've also learned the hard way what happens outside of those limits. Raccoons used to regularly raid our pond for take out sushi. After I electrified the perimeter they haven't touched a fish in many years. We exclusively use our reclaimed sewage water to keep the pond full and the fish flourish in it. We keep the fountain basin fishless and now it's full of wogs soon to become little croakers. :)
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