I am trying to give myself a buffer for water changes. Right now ammonia and nitrites are stable, and nitrates rise as expected. I'd like to know what other steps I can take to reduce the rise inn my nitrate levels.

I have seen these star things which are supposed to drop into the filter, but I'm pretty sure those are for the ammonia and nitrites, as bacteria that eat nitrates don't grow in oxygen rich waters.

I have a 20 gallon long planted community aquarium with a log, a few porous rocks, and an emperor 400 filter (big sale). I'd like to know aside from the plants, what else can I do to reduce nitrogen levels? I'm not trying to eliminate water changes.

1 Answer 1


You basically have 2 options to control nitrates in any fish tank, fresh, salt, or brackish: reduce nitrate introduction (stocking and feeding), or increase nitrogen removal.

The only realistic way to reduce nitrates without water changes in any fresh or saltwater aquarium is through anaerobic breakdown which occurs in anoxic (oxygen free) environments. The problem in a tank this size is that there is virtually nothing that will remove nitrates that is anywhere near cost effective as water changes as it's almost impossible to create the environment that anaerobic bacteria can exist. Even in a 100 gallon freshwater tank it's almost impossible.

Generally speaking there are a few non-water change ways to reduce nitrates:

  • Deep sand beds
  • Large low flowing ceramic or other filtration blocks
  • Sulfur denitrator filters
  • Coil denitrator filters
  • Algae turf scrubbers
  • Algae or plant refugiums
  • Carbon dosing (vodka, vinegar, biopellets, or other), but this method is arguably ineffective in freshwater as it normally requires a protein skimmer to work effectively.

Realistically in a 20 gallon tank, none of these would be remotely worth the cost compared to figuring out an aggressive water change schedule. It would likely take years of operation of another method to break even compared to even daily 100% water changes.

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    I would also add using aquatic plants in the aquarium to the answer. Though most prefer ammonia, some would use nitrates mostly, and for others that prefer ammonia, with little ammonia and nitrites, they'll probably use the nitrates. This link should clarify: www.theaquariumwiki.com/Plants_and_Biological_Filtration
    – Mozein
    Oct 27, 2014 at 4:11
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    @Mozein yea, i'd assume the biological filtration would remove most of the amonia, so they'd have to take N. Oct 27, 2014 at 5:17
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    @Mozein in a 20 gallon tank, you can't stock enough vegetation and a deep enough substrate for either to be effective enough.
    – jwenting
    Oct 30, 2014 at 7:55
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    How would a ceramic filter block reduce Nitrates? As far as I know these are used to convert Ammonia to Nitrites and then Nitrites to Nitrates. So these will actually increase the Nitrate levels in your thank not decrease them.
    – trampster
    Sep 26, 2016 at 1:23
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    @trampster The large ones have enough surface and oxygen depletion to facilitate denitrification if placed in the correct flow area such as a sump. I've personally used a marinepure 8x8x4 block in a 90 gallon reef tank and over time it has been able to maintain unreadable NO3 levels. I do still have to deal with PO4 through another removal method.
    – Jestep
    Sep 27, 2016 at 14:36

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