high Phosphate, low Nitrate - newly cycled tank

[LochNess23]

My nitrate is at ~2 ppm and phosphate is sky high at 0.47 ppm. Any reason for the imbalance and ways I can correct this? Details below:

New tank cycled for 6 weeks with 65x 2” media cubes, 20lbs live rock, 20lbs live sand.
Did a 100% water change, added an additional 10lbs of live rock, and a week later I tested all parameters.
During that week, I’ve only lightly fed twice with sinking pellets and twice with frozen mysis, very small amounts since it’s only 1 damsel fish, 1 cleaner shrimp, and a handful of hermits.
Source water is 7-stage RO/DI with TDS of 0ppm. No other additives or macro algae added yet.

Additional parameters:
ammonia 0ppm (red sea test kit)
nitrite 0ppm (API test kit)
nitrate ~2ppm (nyos test kit)
phosphate 0.47ppm (hanna colorimeter)
pH 7.95 (apex probe)
dKh 10.3 (hanna colorimeter)
temp 77F (apex probe)

I don’t think the phosphate is from the food, as I’ve fed very small amounts and it’s only been a week. Could it be from the live rock die-offs? Or bio media still leaching out phosphate?

 

[Sharkbait19]

Do you know the params of where the live rock came from? It absorbs phosphate, so it’s possible it’s now leeching it into this system.
Also, check phosphate specifically on your source water.

 

[Randy Holmes-Farley Verified Community Expert]

I( agree that rock is often a major source of phosphate in a new tank.

 

[Spare time]

As to how to fix it, just toss a bag of gfo in. It will drop rapidly.

 

[LochNess23]

Do you know the params of where the live rock came from? It absorbs phosphate, so it’s possible it’s now leeching it into this system.
Also, check phosphate specifically on your source water.

Unfortunately I already discarded the water that came with the live rock, so unable to test parameters. These came from TBS.
The bio media is MarinePure 2" cubes from HERE.
I will test the RO/DI water next without relying on the TDS meter.

 

[LochNess23]

As to how to fix it, just toss a bag of gfo in. It will drop rapidly.

So the solution is that simple? Just add GFO until equilibrium and the rocks/media stops leaching?

 

[Sharkbait19]

Pretty much, it’s a lot of water change and chemicals until you get it down. The rocks will continue to leech it out until they run out.
I’ve been spending the last month getting it down from >3 ppm (by a salifert test).

 

[Hooz]

So the solution is that simple? Just add GFO until equilibrium and the rocks/media stops leaching?

If everything else is OK, just use GFO or PhosGuard or something to bring phosphates down without upsetting everything else. It really is that easy. :D

 

[bushdoc]

Yes it could be die-off from the rock. I recently set up new tank with live rock from Tampa Bay, which arrived in water with most of the stuff alive on it, but then decided to add some Aussie live rock. That one arrived wrapped in moist paper and with lots of stuff decaying. I was lucky that sponges and other critters on Tampa Bay Rock survived!

 

[LochNess23]

...Also, check phosphate specifically on your source water.

I just checked RO/DI water. The TDS meter was correct, the Hanna colorimeter was also 0ppm phosphate. Looks like I'll be picking up some GFO until the rocks stop leaching. I only need a small amount, so any difference in quality between these two besides the size?

Seachem PhosGuard
BRS GFO

 

[Sharkbait19]

Phosguard is what I use, but it is known to leech aluminum into the water over time.