New Advice with RO/DI PO4

[mixedreeftank]

I've been battling some green hair algae and brown sand algae working its way up to my rocks. After doing multiple water changes, replacing my metal halide lights, watching how much I feed and changing my Phosguard multiple times, I still have a problem.

My ro/di is a brs 4 stage system. After further investigation and starting testing my tank vs ro/di water vs ro/di water in storage container vs tap water. I am using the the API Phosphate test kit. I know they are not the most accurate test kit, but they do show if there is traces or not.

What I found is that my tank doesn't seem too bad, probably false negative because the algae sucking it up. But my tap water, ro/di water and ro/di water in the container are all the same color. Which is around .5 ppm.

I figured my di resin was due because the color was getting dark. I changed it and I'm still getting the same results.

I'm not sure where to go from here, any advice from others that might have had similar problems?

 

[Xavibear]

I'd get a different phosphate kit & test again.

 

[Randy Holmes-Farley]

Are you monitoring the TDS of the RO/DI?

If it is 0 ppm, you don't have 0.5 ppm phosphate in it. But the DI may be shot even after replacement if it is not designed or hooked up correctly.

 

[mixedreeftank]

I've tested the water multiple times and getting the same results. Going to test some distilled water and compare the three.

My TDS is going in at 3 and coming out at 0. I just replaced the di resin and only a few gallons have been through it. Could my membrane be bad after 6 months?

I know the test kit doesn't give me an exact number but I can tell there is some trace phosphate in the water.

Thank you for the responses guys.

 

[Randy Holmes-Farley]

Trace phosphate in RO/Di is not any concern. Say, 0.05 ppm. That is insignificant relative to the 10-100 times as much that you add each day in foods.

I discuss that here:

Aquarium Chemistry: Phosphate And Math: Yes You Need To Understand Both
http://www.advancedaquarist.com/2012/3/chemistry

from it:

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.

The same sort of calculation applies to analyzing other phosphate issues, such as the GAC in scenario three. The issue of finding "high" phosphate in GAC soaked in fresh water was frequently quoted as a reason to use one or the other brand of GAC, and probably still is. But simple analysis as shown above for the food rinsing puts the lie to this being a big problem.

One needs to consider how much GAC one will really use in the aquarium and how often it is added in order to interpret how important the added phosphate is. A typical recommendation might be 1 cup of GAC per 100 gallons of aquarium water, and to change it in 4-6 weeks. Let's assume we detect 0.5 ppm phosphate when a teaspoon is placed in a cup of water, and we get scared by the dark blue color during the test. Is this reasonable? That 0.5 ppm from a teaspoon in a cup of water translates to 0.015 ppm phosphate when a cup is used in 100 gallons.

That 0.015 ppm may be significant, being a typical target concentration level for reef aquaria and amounting to about half to a twentieth of the amount added daily in foods, but remember, it is used for 4-6 weeks. During those 4-6 weeks before the next replacement, foods add 50-700 times as much phosphate. So while it is not unreasonable to look for another brand of GAC, to blame phosphate or algae issues in the aquarium on its use would stretch credibility because it is a very tiny portion of the total phosphate being added.

 

[mixedreeftank]

Wow thank you for the detailed response.

 

[Randy Holmes-Farley]

You're welcome.

Happy Reefing.