Testing RO water for phosphate

[rmurken]

Just a note to share an interesting pair of results. I used my Hanna ULR Phosphorus checker to test my RODI water, and it came in at 15ppb. I used the same batch of water to mix up 35ppt saltwater, and it tested 0ppb.

Would have to be repeated a bit to become meaningful, but it’s still suggestive. I have read that Hanna discourages using saltwater P-checkersto test FW.

 

[Bleigh]

Just a note to share an interesting pair of results. I used my Hanna ULR Phosphorus checker to test my RODI water, and it came in at 15ppb. I used the same batch of water to mix up 35ppt saltwater, and it tested 0ppb.

Would have to be repeated a bit to become meaningful, but it’s still suggestive. I have read that Hanna discourages using saltwater P-checkersto test FW.

Is it inaccurate for fresh water? I have never thought about this before. I’ve checked my two Ayer using my kits for salt water. Wonder if I should get a freshwater kit for the tap...

 

[Randy Holmes-Farley Verified Community Expert]

I don't know about the accuracy of various kits in fresh vs salt, but in general I do not think it worth measuring in 0 ppm tds water because it won't be the main source in the tank.

I address that here:

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

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.

www.advancedaquarist.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.

 

[rmurken]

I don't know about the accuracy of various kits in fresh vs salt, but in general I do not think it worth measuring in 0 ppm tds water because it won't be the main source in the tank.

Ah, makes sense. Thanks.