Nitrates Constant


Guru Class Expert
Oct 29, 2007
Northern Virginia
Tank stats

72G, EI for 2 months 60-80 gallon guidelines, 260W CF lighting, pressurized CO2 30ppm (DC 4dKH solution), water is 9-10 GH from the tap so I don't add anything but the ferts and traces. No algae issues.

I stopped dosing the KNO3 because my nitrates would climb above 30 ppm. No matter what they seem to stay at 20 ppm (verified with 2 different tests). Fish load is moderate, I suspect it is because of the Wet/Dry filter that has been running with bioballs for the year. I know SW aquarists frequently take out the bioballs because they supposedly cause nitrates.

My concern is that without dosing the KNO3 the plants could become K limited, would it be a good idea to dose more KH2PO4 then the 3/16 recommended? I think I have read that increasing PO4 sometimes encourages the use of NO3.



Lifetime Charter Member
Lifetime Member
Jun 9, 2006
Hi Fred,

How were your plants and fish prior to stopping the KNO3? Any issues?

Are you doing weekly water changes? If so this build up doesn't seem possible.

I don't think bio balls cause nitrates. They do help ammonia and nitrite get to that point, ie bio filters.



Guru Class Expert
Oct 29, 2007
Northern Virginia
No problems before hand, I just tested and saw I had more then enough nitrate naturally for whatever reason. First it comes out of the tap at 10., so even with a 50% water change you go from 20 to 15, not much of a change really. I'd like to induce the plants to use more NO3.

Cause I think was a poor choice of words. They wet/dry are extremely efficient biofilters. In salt systems the accepted norm is typically either take out the bioballs and run without or use live rock rubble in lieu of.

My main concern is limiting K, b/c it does not really occur naturally in tanks.



Lifetime Charter Member
Lifetime Member
Jan 24, 2005
Sacramento, CA
Why would you want to limit potassium? Plants use about the same amount of potassium as nitrate, but there isn't a critical amount needed. And more than enough potassium has never, as far as I can remember, been suggested as a problem in an aquarium.

There isn't much commonality between salt water aquarium care and planted tank care, other than that both benefit from good cleanliness standards and good water circulation - again as far as I can remember from the reading I do.

Don't assume your nitrate test kit is giving you a good reading unless you calibrate it with water samples of known nitrate concentrations. Our test kits are not laboratory quality, and even if they were, laboratory testing is always preceeded by calibrations.


Guru Class Expert
Oct 29, 2007
Northern Virginia
I don't want to limit potassium. I was just concerned that since I was not dosing KNO3 because I believe NO3 to be high enough, that I was in essence limiting potassium which I did not want to do. I thought maybe I could offset by using a higher dose of KH2PO4. How far off can the hobbyist test kits be? I am using the API one, and coroborating with strips (which I know are inaccurate).

Basically I have young angelfish, and some Diamond tetras in the tank that I bred and I want to encourage to grow and do not want the nitrates to get too high.

Tom Barr

Staff member
Jan 23, 2005
In marine tanks, there's not much NO3 export and bioballs do nothing for that, only NH4=>NO2=> NO3 really good and add O2.
Otherwise they really have little function.

Adding a massive skimmer which really adds plenty of O2+ removes a lot more waste products, helps a lot more.

The live rock is a bunch of baloney, yea, you get some critters, but bacteria is easy to grow and that is what and why folks add it.

The very fine pores etc help to NO3=> N2 gas, denitrify the water.
That is why they add that + a skimmer and no Bioballs.
That reduces water changes.
Which is their goal.

Nothing wrong doing this with FW tanks either, I do.
As well as for marine tanks.

I use a sock bag filter, sump etc and a bunch of media(larger Zeolite, SeaChem denitrate etc etc).

Same type of concept.
No skimmer though.

The main issue: sealing up the splashing area at the bag filter location so all the CO2 gas is not lost. Same with bioball sections. Plug all the holes up so it does not degas, but rather, goes back into solution.

Get some good CO2 going, some K2SO4, you should knock the NO3 down pretty quick.

I'd not rely on readings from 3 test kits, rather, use the reference solutions only.
I have some nice equipment, a 3000$ spect, but I still use a ref calibration set to make sure.

Why use a test method when you are not sure about it?
I can look and do a water change easier than making such guesses.

Either do the calibration, or do not use the kit.
There's no 3rd choice there.
Some seem to think so.

I suppose if the method had shown 5-10X that it's as accurate as you feel needed for your purposes, then sure.

Lamotte fit that bill quite well.
Same for Hach.

One thing if the NO3 suddenly slow down the uptake rates, do these things:

Really check the CO2.
Clean the filter, clean/prune plants, light vacuum, maybe even uproot some sections and deep vac if it's been 1-2 years since.
Big water changes, clean everything 2x a week for 2-3 weeks and see.

That tends to take care of most issues folks have.

Tom Barr