
Originally Posted by
Urban Aquaria
Its possible the nitrates are just being generated faster than your water changes and Purigen can remove... probably it climbed up to 80ppm over a week (since your last test), then when you did the recent 50% water change, it halved back down to 40ppm.
Next round, maybe try testing in the water before and then immediately after the water change to see if it was effective in reducing the nitrate levels. Note that in some water supplies, the tap water has some nitrate content too so it'll affect the results.
The higher nitrate levels in your tank could be a combination of higher soil to water ratio (as you mentioned you have a relatively deep aquasoil substrate for the tank size, hence more nitrate build-up), and perhaps not enough fast growing plants to take up sufficient nitrates to keep the levels in check.
I guess you could plant more fast growing plants, crank up the Co2 to help the plants photosynthesize more (but still within safe levels for the livestock) and do more water changes to flush out the excess nitrates. Eventually the soil's nitrates will deplete and you'll see the levels decrease accordingly.
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