Nitrate Removing Filter Pads.

Some people report good results with API Nitra-Zorb. JBL NitratEx is also recommended, though one person has commented that it becomes less effective after being recharged numerous times.

Don't bother with things like Juwel's green sponges, it is generally accepted that they don't work.



Years ago there was something called nitrex. This was black, almost rubbery granules. It was put into a special flat box; the lid sloped gently and there was a slit along the highest edge. You buried the box under the substrate with just the slit showing. The idea was that the water inside the box became depleted of oxygen and anaerobic bacteria grew on the media which used nitrate as food, turning it into nitrogen gas which escaped through the slit as bubbles. I don't know if there is anything like this available now.
The problem with biological methods to remove nitrate is that the bacteria which do it don't like oxygen which is why they won't grow in the filter.