I just wrote this in another thread, so I'll just copy and paste:
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The answer to your question is that there are, basically, 3 ways to reduce nitrates:
1. "The solution to pollution is dilution", meaning do water changes.
2. Grow some anaerobic bacteria to convert the nitrates into harmless gaseous nitrogen, meaning use a Deep Sand Bed (DSB).
3. Grow something much larger than bacteria that consumes nitrates for food, like cheato, which you grow in your sump, not in your DT.
Oh, and there is a 4th thing, but it's not how to reduce nitrates. This 4th item is how not to produce so much of it in the first place.
4. Eliminate your "nitrate factories", which is to say, change and clean your filter material more frequently.
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PS- My glass is empty. Who's got the bottle?