How high can i let my nitrates go

[svc]

I realize that nitrates are the least toxic form of nitrogen but... my readings are off the scale even after a 50% water change. My kit tests to 100mg/L so i am assuming i was at least double that before a water change. Ammonia is 0 and i don't have a nitrite test yet. I have approx 12 goldfish in a 65L tub. Another 65 L tub is used for flood/drain growbed. Plants are all still seedlings, so i am expecting them to eat more nitrate in the future. Does anyone have a general guideline figure for how high nitrates can go before fish are affected?

[Ravnis]

Most studies I have read say fresh water fish can handle 400 ppm.

100 ppm is generally a concern to crustaceans and mollusks. You dont list what fish you are growing. This is important as one species has different tolerances than others. So general advice may not apply. I have noted in the past problems with keeping nitrites at zero with nitrate about 180. So that would be the main concern.

Crusty has more experience in this matter, so will defer to his knowledge, but I personally would not bother changing water with only 100 mg/L of nitrates unless nitrites were staying high.

[kellenw]

Svc,
You might also want to keep an eye on your pH. Also, are you getting any algae growth in your system? You will typically get an algae bloom when you have excess Nitrates. Often, pH will begin dropping when you have excess Nitrates and an algae bloom. With an Ammonia reading of 0 and detectable Nitrates in the system, it is highly likely that you will have a Nitrite reading of 0, but test to be sure. I would not worry about your current Nitrate level with goldfish. If you wish to decrease it without water changes, add some bog plants or aquatic plants to the fish tank. However, you need to be careful not to remove so much that your GB seedlings are starved.

Ravnis - I think you missed where he said he has a dozen goldfish in a 65L tub. ;)

[Ravnis]

Ravnis - I think you missed where he said he has a dozen goldfish in a 65L tub. ;)

Heh, guess I'm getting blind in my old age.

Goldfish are very hardy btw, really hard to kill them, as they survive conditions that will kill tilapia.

[Crusty]

Looks good to me. Most titration kits don't read over 160mg/L. I will only add, avoid nitrates above 300mg/L (ppm) for extended periods (weeks). Your plants will like you keeping nitrates at >100mg/L. As already said, there is no need to change out water.

[svc]

Thanks for all the replies. The system is new so no Algae yet. I will start adding 3/4 tap water when i test to get the range to 400.

[Crusty]

Thanks for all the replies. The system is new so no Algae yet. I will start adding 3/4 tap water when i test to get the range to 400.
If you are planing to dilute the water from your tank to be able to read higher than the titration test will go, then you may need to test the tap water for nitrate (doubtful). The difficult part of that is the water will be contaminated with other chemicals which will give you a false reading. It may be easier to use 10ml of testing water to the drops for the titration test instead of 5ml then just double the reading, 20ml and times by 4 etc...

If you have high nitrate and you have a low oxygen (more than likely) the reason for keeping the nitrate around the 300mg/L mark is because you will have anaerobic zones in your grow beds and the bacterial reduction of nitrate to nitrite will be happening as part of the denitrification process. In other words, your will go from (nitrification) ammonia to nitrite to nitrate back to (denitrification) nitrite. If you need kits, I have them here.

[svc]

If you are planing to dilute the water from your tank to be able to read higher than the titration test will go, then you may need to test the tap water for nitrate (doubtful). The difficult part of that is the water will be contaminated with other chemicals which will give you a false reading. It may be easier to use 10ml of testing water to the drops for the titration test instead of 5ml then just double the reading, 20ml and times by 4 etc...
.

I will try that too. I imagine that the color will be the same (just a lighter shade). I don't know how the test works though so i am more than likely wrong.
Thanks again for the help. I think I will feed a few goldfish to my mates Oscar if levels stay high, I can always add more later when the growbed establishes (I have another GB to add also)

[svc]

Update : after a week of feeding less, nitrates are reading 30 - 50mg/L. There is no where for anaerobic bacteria to colonize (none i can think of anyway) so it must be the plants. Woohoo it all seems to be working 'as advertised'