algae in a 172g koi tank

One of Chicago's western suburbs...near Naperville.

I knew we had some phosphates in the tap water, but I didn't realize the levels were that high!
 
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They're not usually. Like I said the highest I remember ever seeing was about 0.5 ppm. Municipalities with aging distribution systems add zinc orthophosphate to their water as a corrosion inhibitor. This keeps the water from ripping off all the scale and 'junk' that has built up in the water lines over the years. Unless of course, you're on a well, then you could be talking about ground water intrusion (run-off from farm fields).

In any case, I hope you find the answer to your problem.
judy
 
The one time i had green water it was due to ammonia from over feeding. Once i had ammonia under control, i used a diatomfilter to remove the stuff and it never came back.
 
> Municipalities with aging distribution systems add zinc orthophosphate to their water as a corrosion inhibitor.

That could easily be the source of it... since we're on the Chicago area water system. The water's from Lake Michigan (yup, piped over 30 miles).
 
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