Nitrogen Cycle question

[MontanaBay]

Hi All,

I've had a tank cycling for about 5 weeks now, started with dry rock and sand and a cocktail shrimp. The shrimp as dissolved away into nothing, I seeded the tank with my established tanks water, some sand, and LR rubble. I didn't check any parameters for the five weeks, just waited for the shrimp to disappear.

Last week I checked the parameters:

ammonia= 0 ppm
Nitrite= 0 ppm
Nitrate= 160 ppm

So I did a 100% water change and got:

ammonia= 0 ppm
Nitrite= 0 ppm
Nitrate= 60 ppm

so I did another 100% water change and got

ammonia= 0 ppm
Nitrite= 0 ppm
Nitrate= 20 ppm

I'm about to do a third 100% change, but I'm wondering why my nitrates aren't dropping out? Test my source water, which is 0 ppt, but why is this taking so long to come down? Do the rocks and sand absorb some and slowly release?

Thanks,

Josh

 

[pixelpixi]

What do you mean exactly by "100%"? Do you mean that you completely drained the tank and then filled it with new water?

 

[MontanaBay]

not the 2" left in the sand and whats left in the live rock, so maybe 90% on each change.

 

[A_Lee]

If you're looking for an easy way to cut the nitrates down, close to zero, I'd recommend just throwing a decent sized ball of chaeto into your refugium (or directly into your DT if a refugium is not present). Be sure it gets good light and flow, and your nitrates (and PO4's) should be tons lower after a couple weeks. Plus, you will save salt mix and water, and labor!