what is happening to the phosphates here?

[jarviz]

So I have a second tank that i have been experimenting with. It's a FOWLR tank 5 gal. I started this with NYC tap water. TDS is ~40ppm, so relatively clean for tap water. Tank is about 3 months and went thru a recent diatom but no other major algae issues. I started with dry rock and "live" sand. I read about 0.17 for Phosphates with a hanna checker. I was ok with this and just wanted to see where it would go. However, when i tested a fresh batch of saltwater mix. I picked up 1-2ppm, maybe even 5ppm phosphates with an API test. Hard to tell tho with the colors.

Why would phosphates be lower in the tank? I do have phosguard, but i can't imagine it bringing down the numbers that much. I tested a fresh batch of saltwater i used with my RO/DI kit (just for comparison) and got 0.01ppm.

I was reading some report that NYC's water resorvoir was around 15ug/l, which is supposedly around 0.015ppm. I could be wrong tho as it was a convoluted report. Do these numbers make sense?

 

[Randy Holmes-Farley Verified Community Expert]

Phosphate can be very high in tap water. The last new york city water report I reviewed (2002) showed 0 to 5.4 ppm phosphate:

Aquarium Chemistry: Tap Water in Reef Aquaria

As will become clear in this article, using tap water is risky business. Short of testing your own water, there is no way to be certain that your water is acceptable.

reefs.com

Phosphate can also bind strongly exposed calcium carbonate rock, potentially reducing the concentration by many ppm.

 

[jarviz]

Phosphate can be very high in tap water. The last new york city water report I reviewed (2002) showed 0 to 5.4 ppm phosphate:

Aquarium Chemistry: Tap Water in Reef Aquaria

As will become clear in this article, using tap water is risky business. Short of testing your own water, there is no way to be certain that your water is acceptable.

reefs.com

Phosphate can also bind strongly exposed calcium carbonate rock, potentially reducing the concentration by many ppm.

so if i swapped back over to RODI water for this tank. the phosphate levels may not drop for a while till it all comes off the rocks right? I was thinking of just doing 20% weekly water changes with the RODI. Or does high phosphates not really matter for a FOWLR tank

 

[jarviz]

I also read that water changes are not great at reducing phosphates? Does that apply in this case were my starting water is tap water and i perform WC with rodi water?

 

[Randy Holmes-Farley Verified Community Expert]

Bare rock surfaces tend to try to hold the phosphate concentration where it is. Hard to raise and hard to lower.

If it is only 0.17 ppm now, then that will be an effective starting point (for raising or lowering) and is OK.

Correct that water changes are not especially effective at exporting phosphate in a situation like this.