Not totally sure why your nitrates stay high, but I would start by verifying that your test kit is giving you good readings. Could you verify what you run inside the canister?
From there, you could look at chemical/filter based methods of reducing them. Several companies offer liquid nitrate reducing products. Biopellets are also very popular and can run from an external fluidized reactor for a pretty reasonable price.
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Mark...
290g Peninsula Display, 425g total volume. Setup Jan 2013.
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