Phosphate test results varying day to day

[PlantedReef]

I have a 8 month old 40g breeder with 40lb of rock, 2" sandbed, mixed corals (mostly softies\LPS\4 SPS) and about 8 fish. Also using 2 AI Prime 16HD LEDs. I have been trying to raise the level of nutrients in my mixed reef tank from basically zero for both nitrate and phosphate. My corals look great but, have fairly slow growth. I feed Polyp Labs polyp booster, reef roids (1-2 times/wk and Fuel, Seachem zooplankton and Seachem phytoplankton (twice a week). My latest readings are as follows:
Nitrate - less than 1ppm (Nyos test kit)
Phosphate - .05 (Hanna test kit)
ALK - 8.0
Mg - 1320 (Salifert test kit)

For the last month, I've been trying to raise my nitrate and phosphate to try and battle dinoflagellates that are starting to creep up on rock and sandbed. And to just get them to a healthier level. Nitrates are very steady at less than 1ppm. My real question here is why my phosphate readings are all over the board. I take a reading everyday at approximately the same time. Fish feedings are done in late morning and late evening using mainly herbivore flakes and regular saltwater fish flake food (sometimes frozen mysis and Rods frozen foods). I'm testing my parameters the next day prior to any feedings.

Here are some examples:
8/18 10:30am Nitrate less than 1ppm Phosphate - 0.00 (Hanna)
8/19 10:30am Nitrate less than 1ppm Phosphate - 0.13 (Hanna)
8/20 1:00pm Nitrate less than 1ppm Phosphate - 0.00 (Hanna)
8/21 2:30pm Nitrate less than 1ppm Phosphate - 0.00 (Hanna)
- SHUT OFF PROTEIN SKIMMER
8/22 2:00pm Nitrate less than 1ppm Phosphate - 0.00 (Hanna)
8/24 1:00pm Nitrate less than 1ppm Phosphate - 0.00 (Hanna)
8/26 1:00pm Nitrate less than 1ppm Phosphate - 0.16 (Hanna)
- STARTED BUBBLE SCRUBBING IN THE EVENING
8/27 12:30pm Nitrate less than 1ppm Phosphate - 0.05 (Hanna)

Why are my phosphate readings all over the board when I'm doing my testing hours after any feedings and approximately at the same time of day?

[jeremy007]

Fish poo?

[Vinny Kreyling]

Test kits have a margin of error. Read the manual to determine what yours is but I would bet the farm you are within those parameters.

[outssider]

the phosphate checker (H713) is not as accurate as the phosphorus checker (HI736) ! especially at very low levels of phosphate, it also uses ppb rather than ppm

[Timfish]

Everything in your tank is messing with phosphates (and nitrogen and carbon). Sponges are sequestering it and releasing phospholipids back in the water. Biofilms are sequestering it and as cryptic fauna and and clean up critters feed off it it's dumped back into the system. ANd don't forget corals and algae. I would recommend not chasing it daily. What I would consider more important to track would be alkalinity, calcium, magnesium but unless there's a problem I would check those weekly.

[PlantedReef]

Originally Posted by Timfish

Everything in your tank is messing with phosphates (and nitrogen and carbon). Sponges are sequestering it and releasing phospholipids back in the water. Biofilms are sequestering it and as cryptic fauna and and clean up critters feed off it it's dumped back into the system. ANd don't forget corals and algae. I would recommend not chasing it daily. What I would consider more important to track would be alkalinity, calcium, magnesium but unless there's a problem I would check those weekly.

Thanks for the feedback. Great advice. I'm letting the tank tell me when my nutrients are getting a little out of control by how dirty the glass and sand gets over time. Check nitrates and phosphates every couple of weeks now.

[PlantedReef]

Originally Posted by outssider

the phosphate checker (H713) is not as accurate as the phosphorus checker (HI736) ! especially at very low levels of phosphate, it also uses ppb rather than ppm

Good info. Yes, I am using the HI713 phosphate checker. I keep reading that 0.03 is around where you should be shooting for but, my results are typically 0.00. If I test daily, they are all over the place. Hard to get to a specific number when tests range from 0.00 - 0.18 constantly. I don't test as often anymore. Usually once a week or so.

[Sugar Magnolia]

There's no need to really worry about it, testing weekly is what most people do. The important params to keep in check are calc, alk and magnesium along with salinity.

[Timfish]

Originally Posted by PlantedReef

Thanks for the feedback. Great advice. I'm letting the tank tell me when my nutrients are getting a little out of control by how dirty the glass and sand gets over time. Check nitrates and phosphates every couple of weeks now.

Thank you! If you're interested here's some videos you might find informative:

Forest Rohwer "Coral Reefs in the Microbial Seas"
https://www.youtube.com/watch?v=-R2BMEfQGjU

Changing Seas - Mysterious Microbes
https://www.youtube.com/watch?v=c7hsp0dENEA

Nitrogen cycling in hte coral holobiont
https://youtu.be/DWItFGRQJL4

BActeria and Sponges
https://www.youtube.com/watch?v=-oLDclO7UcM

Richard Ross What's up with phosphate"
https://youtu.be/ZRIKW-9d2xI

[chiefifd]

I was having the same issue a few years ago using the Hanna HI713 tester, can be confusing for sure. I'd definitely buy the Hanna HI736, Hanna does make a newer HI774. The HI774 gives you a phosphate reading in PPM, the HI736 gives you a reading in PPB. the Hanna HI736 Ultra Low Range Phosphorus Checker measures in parts per billion rather than PPM. The HI736 has a range of 0 to 200 PPB, to convert phosphate you will need to multiply the reading by 3.066 and then to translate that to parts per million, divide that number by 1,000. You get used to the HI736 readings pretty quick, example a reading of 52 with the HI736 = 0.159 PPM.
There's a chart online here somewhere that gives you the conversion without doing the math. Good luck

[Timfish]

Keep in mind the Hanna low range phosphorus checker is actually testing phosphates, PO4 or dissolved inorganic phosphorus, and converting the reading to the equivilent amount of phosphorus. Since it's not testing dissolved organic phosphorus or particulate phosphorus the total phosphorus in a system will be more than what the checker says.