Help reducing nitrates


New member
I am looking for advice in how to reduce the nitrates in my tank which are currently at 80ppm. A few weeks ago they were well over 100ppm and I did a 30% water change. Here are the specifics of my setup:

450 gal hex tank on main floor of my house
150 gal sump in the basement of my house (appr. 50 ft of dual PVC pipe between tank and sump)
Dual ATS system (dump style) above the sump in the basement
Apr. 100 fish
-- Foxface Rabbit, Magnifcent Rabbit, Scribbled Rabbit, 4 Hepatus Tang, 4 yellow tang, 2 Leopard wrasse, solorensis wrasse
-- Colony of dispar anthias
-- Colony of threadfin cardinals
-- 2 watchmen gobies + shrimp
-- pipefish
-- colony of blue damselfish
-- 4 clownfish + 4 red bubble tip anemones
Mostly soft corals (have not been successful with hard corals)
2 large clams

Tank is full of live rock with a 5" sand bottom.

I am feeding twice a day, each feeding consists of 2 rotations from autofeeder containing 80% flake (New Life Spectrum Optimum), 20% pellet (Sustainable Aquatics Hatchery Diet) food. I also feed frozen food appr every other day (Rod's). And a couple times a week I feed a small sheet of green or red seaweed.

I scrape the ATS screens once a week and thoroughly scrape the dump box. Lights on the ATS screens stay on all of the time.

I also have a combined GAC/GFO reactor which I currently completely change the media every other week. Media consists of 4 cups GAC, 2 cups high capacity GFO in a packed bed arrangement. This has controlled phosphate which used to be much higher. Currently at 0.17 ppm which I realize is still too high for hard corals.

Tank has been in operation for appr 15mo and I've learned several lessons along the way. But the nitrate issue that I'm currently experiencing is really confounding me. Any advice/recommendations are highly appreciated.


Premium Member

How much live rock is in the system? With that many fish, I think that the nitrate level probably is about what I'd expect. It won't harm the fish, so it could be ignored.


Premium Member
That's a fairly small amount of rock for such a tank. You might be able to reduce the nitrate level by using a variety of techniques, but the animals you listed aren't going to care. If you'd like to try, you could look into vinegar dosing:

That's an easy place to start. A larger ATS or more lighting or some phosphate dosing might help, too, depending on what's limiting the growth in your current setup.


New member
I am looking for advice in how to reduce the nitrates in my tank which are currently at 80ppm. A few weeks ago they were well over 100ppm and I did a 30% water change. Here are the specifics of my

The nitrate concentration is too high to accurately judge the level. Dilute 1 mL of tank water with 99 mL of RO/DI water. This will reduce the level to 1/100 and allow for a more reliable estimate. If 80 ppm is correct, the diluted sample will be 0.8 ppm.

If the original 100 ppm is correct, a 30% water change should have reduced the nitrate level to 70 ppm. You measured 80 ppm. Not bad. Run the dilution experiment anyway.

Another point is that if the nitrate level is holding at 80 ppm and not bouncing back to 100 ppm, this would indicate that your system could be at steady state. It is removing nitrate as fast as it is producing it. The simple solution is to continue water changes. But if that is too daunting, you will need to increase the system's dentrification capabilities. The least intrusive to the aquascaping is carbon dosing. In any case, I wouldn't be too quick to change your system.


New member
Thanks for the suggestions. I ran another nitrate test, diluting 1:10 since the API test kit I'm using is higher range. I found the nitrate to be around 75ppm, interpolating the colors. I'll continue to monitor to judge steady state.

At what nitrate levels should I be concerned? I have not lost a fish for several months indicating the fish have acclimated to the current nitrate levels. However, if I introduce new fish, is it likely that nitrate shock could ensue?

I'm considering adding a matted filefish to control aiptasia. Not sure if I should focus on reducing nitrate to prevent issues with newly introduced fish.


New member
In general fish don't seem to be bothered by elevated nitrate levels like yours..
I've seen them doing just fine as far as one can tell in 100ppm+ tanks...

No one really knows that they "magic" number and its different for each fish really and how old they are,etc... but there are numerous tanks with levels like yours doing just fine...

I would reduce it because of the other issues like increased aiptasia, algae and cyano issues,etc...

And if you want anything but soft corals then you will need to get it down..
I personally have found that its much easier to keep corals when nitrate levels are below 10..

At this point your input seems to be greater than your output (or its stable) so you need to increase your export methods... That could be increasing your ATS system or carbon dosing or increasing the amount of rock,etc...
Many have found carbon dosing (just vinegar is fine) to be a very effective way to control nitrate levels..


Premium Member
The lowest numbers I've seen for nitrate toxicity in fish were over 600 ppm, and I believe that was juvenile fish. Personally, I would ignore the level as long as the tank is doing well enough.

Peppermint shrimp (Lysmata wurdemanni) will control Aiptasia, although making sure that you get true L wurdemanni can take some effort. They won't add filtration load, which is an issue if you want to keep the nitrate level down.