Denitrate Media Not Working : Solved !

  • #1
HI stop reading now if you care about having low phosphates in your tank.

I set up a ten gallon tank two months ago. Stocked with 15 juvenile guppies. Two problems in my tank. Nitrites and nitrates off the charts. Battled and battled it with 80% daily water changes. (Ammonia has always been at zero since week 4)

I then added denitrate media to my 2 litre, 45 gph , external canister filter. Hyped up by the blurb on the denitrate label I sat back and waited two weeks for the magic to happen. Nothing happened.

Nitrates and nitrites were still through the roof. pH was hanging around 8.0 to 8.3.

I read and read about the nitrogen cycle bacteria. Learned that optimal pH is around 7.5.
Only pH down to hand was food grade phosphoric acid.

Gradually added this acid to the tank over a couple of days.

pH now 7.5 . Reading with liquid test kit : Ammonia 0, nitrites 0, nitrates 10

Turns out my tap water has next to no phosphates. Problem was something I barely read on forums for beginners: phosphate block
Cured by phosphoric acid.

I wonder how many new aquarium owners, in a fit of paranoia, are stripping phosphates from their tank water? Then wondering why the cycle is stalling....

It is pure luck I had food grade phosphoric acid to hand.
  • #2
Your pH is fine, don't try to change it. Changing it can be more dangerous.

And the cycling problem could easily be from you adding 15 guppies to a 10 gal tank. Denitrates don't usually work in normal filter setups, so I'm guessing your water changes reduced it and your cycle finally caught up to reduce nitrites.
  • #3
There's plenty of phosphate in fish food. I thought it might've been the problem years ago when I was using RO/DI water so I tested phosphate levels and they were actually kind of high.

Nitrification requires small amounts of phosphate. I haven't been able to find anything to suggest that denitrification also requires phosphate.

You would expect a build up of nitrite rather than nitrite and nitrate but I suppose it's possible.

Denitrifcation isn't easily achieved in freshwater. Nitrite and nitrate tests are similar and not that reliable. They're also subject to interference from Amquel/Prime.

I wouldn't draw any conclusions just yet.
  • Thread Starter
  • #4
I took that advice about not chasing the pH from this forum.

Ended up in deep trouble with a stalled cycle due to high pH and phosphate block .

Problem cleared 36 hours after adding phosphoric acid, dropping the pH to 7.5.

Maybe another test would be to allow pH remain above 8 and add phosphates?

Or use a pH down that does not contain phosphorus compounds ?

Happy to have a fully cycled tank now. Not really ready for another experiment just yet....
  • #5
These de-nitrate media, Marine Pure media, basically any media that helps with reducing nitrates and phosphates is not achieved within weeks.

To get brand new media going with just some BB colony takes a solid 2-3 months. These de-nitrifying bio-media takes twice the duration to see any noticeable difference.
  • Thread Starter
  • #6
The tank was seeded using materials from a well established tank.
It is truly bizarre how quickly it settled after battling for weeks to keep NO2 / NO3 under control.
The new media was sitting in the filter with older established media for at least two weeks before the acid was added.

The acid is the only thing that changed things in 36 hours
  • #7
Your pH is not too high to properly cycle the tank. I have a pH of 8.2 in all of my tanks, and they all cycled and have stayed that way, with no issues. Something else, possibly the phosphate issue, caused it, not your pH. It's not prohibitively high under normal circumstances.

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