I have to disagree with Rodney on both the water change and nitrates in a
green pond. One serious koi keeper said his nitrate test was not working as
he should have loads of nitrate as his pond was green. I asked if I could
have it to test on my aquariums, that always run a readable nitrate level,
and sure enough it work perfectly even though the aquariums are clear.
A green pond has little to no nitrate reading because the green algae is
consuming it.
2nd water changes. It is very rare that a USA municipal water source has
high nitrate or phosphates. But that is a good point that Patrick needs to
take into consideration, where is his fresh water coming from? If city,
there will usually be on-line information regarding his water.
Many serious koi keepers, the ones that spend the big bucks... and some not
so big are doing the slow tickle in, old out, on a constant year around
basis and finding it doesn't cost any more than the 25% every week that is
a standard recommendation.
One really good article on Green Water, long, but worth it:
http://koiclubsandiego.org/library/green_water.php ~ jan
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Zone 7a, SE Washington State
Ponds:
www.jjspond.us