Phosphate reading of .12 in new salt water.

[Dom]

Admittedly, I have tested my phosphate level in my tank for the first time in a long time. Using the Hanna ULR, I came back with a reading of .48.

I placed 1000ml of RODI in a beaker and brought the salinity up to 35ppt. Then I tested and found that my new salt water has a phosphate of .12.

Have my weekly water changes continued to add to the phosphate level and cause the .48 reading?

I am using a 5 stage BRS RODI system whose filters were changed June of 2024 (6 months).

So while I have identified the source of the phosphates, I don't know where in the RODI system the problem lies, but, I do have an idea...

A few months into the new filters, I wasn't getting any output AT ALL from the RODI. I figured out that the first of the two carbon block stages was completely clogged. I removed the carbon block and started up the system with an empty stage. The water flowed fine.

So in an effort to preserve the remaining filters, I used a DI resin canister packed with polyfil as a pre-filter and installed it in the position of the clogged carbon block. It seemed to work very well. But I am wondering; has this become the source of the phosphates?

 

[Randy Holmes-Farley]

If the TDS is 0 ppm, I'd ignore it. If it is higher, I'd replace the DI.

Aquarium Chemistry: Phosphate And Math: Yes You Need To Understand Both

Foods are by far the most important source of phosphate in most aquariums. In considering whether sources of phosphate other than foods are important, one must carefully look to the actual amounts involved to determine whether other sources are even worth trying to minimize.

reefs.com

Comparison of Food Sources of Phosphate to Other Sources
What about other sources of phosphate, like the “crappy” RO/DI water containing 0.05 ppm phosphate? A similar analysis will show it equally unimportant relative to foods.

Let’s assume that the aquarist in question adds 1% of the total tank volume each day with RO/DI to replace evaporation. Simple math shows that the 0.05 ppm in the RO/DI becomes 0.0005 ppm added each day to the phosphate concentration in the aquarium. That dilution step is critical, taking a scary number like 0.05 ppm down to an almost meaningless 0.0005 ppm daily addition. Since that 0.0005 ppm is 40-600 times lower than the amount added each day in foods (Table 4), it does not seem worthy of the angst many aquarists put on such measurements. That said, tap water could have as much as 5 ppm phosphate, and that value could then become a dominating source of phosphate and would be quite problematic. Purifying tap water is important for this and many other reasons.

 

[Dom]

If the TDS is 0 ppm, I'd ignore it. If it is higher, I'd replace the DI.

Aquarium Chemistry: Phosphate And Math: Yes You Need To Understand Both

Foods are by far the most important source of phosphate in most aquariums. In considering whether sources of phosphate other than foods are important, one must carefully look to the actual amounts involved to determine whether other sources are even worth trying to minimize.

reefs.com

Comparison of Food Sources of Phosphate to Other Sources
What about other sources of phosphate, like the “crappy” RO/DI water containing 0.05 ppm phosphate? A similar analysis will show it equally unimportant relative to foods.

Let’s assume that the aquarist in question adds 1% of the total tank volume each day with RO/DI to replace evaporation. Simple math shows that the 0.05 ppm in the RO/DI becomes 0.0005 ppm added each day to the phosphate concentration in the aquarium. That dilution step is critical, taking a scary number like 0.05 ppm down to an almost meaningless 0.0005 ppm daily addition. Since that 0.0005 ppm is 40-600 times lower than the amount added each day in foods (Table 4), it does not seem worthy of the angst many aquarists put on such measurements. That said, tap water could have as much as 5 ppm phosphate, and that value could then become a dominating source of phosphate and would be quite problematic. Purifying tap water is important for this and many other reasons.

So the total TDS in my in my storage container is 1. To your point, the DI resin isn't the problem.

Are phosphates cumulative?

If my tank has a PO4 reading of .4 and my top off has a PO4 of .1, shouldn't my phosphates come down?

 

[Randy Holmes-Farley]

So the total TDS in my in my storage container is 1. To your point, the DI resin isn't the problem.

Are phosphates cumulative?

If my tank has a PO4 reading of .4 and my top off has a PO4 of .1, shouldn't my phosphates come down?

No, because top off is replacing evaporated water that removed no phosphate by evaporation. Water out and water plus phosphate in.

 

[Dom]

No, because top off is replacing evaporated water that removed no phosphate by evaporation. Water out and water plus phosphate in.

So this is like salinity in that water evaporates, not salt.

In this case water evaporates, not PO4?

 

[Randy Holmes-Farley]

So this is like salinity in that water evaporates, not salt.

In this case water evaporates, not PO4?

Correct.

 

[BubblesandSqueak]

might be a dumb question but did you test your phosphates in just the RODI water?

 

[Dom]

might be a dumb question but did you test your phosphates in just the RODI water?

No; I tested PO4 in the display tank and found it to be high. I tested the RODI to locate the source of the PO4.

 

[BubblesandSqueak]

No; I tested PO4 in the display tank and found it to be high. I tested the RODI to locate the source of the PO4.

right but the mixed rodi/salt you measured in first paragraph. So was just wondering if it is the salt or the DI water that has the PO4. I would assume salt but just rules out that the RO was ok?

 

[Dom]

right but the mixed rodi/salt you measured in first paragraph. So was just wondering if it is the salt or the DI water that has the PO4. I would assume salt but just rules out that the RO was ok?

Oh... I understand what you're asking...

So I mixed up salt water to test PO4 because it was a saltwater test kit. Ideally, I should test JUST the RODI.

 

[Randy Holmes-Farley]

I personally wouldn’t bother retesting. Its not the primary source of phosphate.