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Generally speaking, elevated nitrates are not a problem and expected in an octopus tank. Like any excess, however, there is a limit to how high they can be before there is a negative impact. Sadly, we don't have a definitive answer to that number.
Going along with the point DWhatley made, I usually do water changes for anything above 20-30ppm for Nitrates. Anything much higher than that and you are likely stressing out your animal.
There is something to be said however about maintaining a balance of good water chemistry, and not doing water changes too often or swinging the water quality parameters too quickly. Depending on your specific situation and the actual measure of Nitrates in your tank I can give more specific advice.