(MC) wrote:
I brought my Koi indoors for the winter and am experiencing high
nitrate levels although I believe I have sufficient filtration. I have
read this isn't a big problem in some places, but on the test strips
and elsewhere read it was unsafe. I have about 100-150PPM. Is this
something to worry about? Nitrites are almost non-existent and PH is
fine.

Yeah, that's a problem. Start doing water changes to drive it down to something
more reasonable (BTW, filtration does very little for nitrates -- they're the
end product of the nitrogen cycle. You either have to have lots of green
growing stuff to suck up the nitrates, or do water changes to dilute them).