RO/DI not removing any phosphates?

[ClaireCoral]

I am running an AquaMedic 4 stage RO/DI system, which had the DI resin and membrane replace only a few months ago. For a while the TDS meter would flash 99 after flushing for a few minutes, then drop and stay around 03-04, but never reached 0 even directly after replacing the filters. I sort of figured that the meter might be off.
The issue I'm facing now is that the TDS meter is back to flashing 99 at all times and the phosphates directly from the clean water output are reading 2ppm, on an API freshwater tester, which is the same ppm reported in my local tap water. My DI resin is still mostly brown, its the color shifting variety, and I didn't think a membrane would need to be replaced so soon. Does any one have any advice, or have dealt with something similar?

 

[FishLvR]

I am running an AquaMedic 4 stage RO/DI system, which had the DI resin and membrane replace only a few months ago. For a while the TDS meter would flash 99 after flushing for a few minutes, then drop and stay around 03-04, but never reached 0 even directly after replacing the filters. I sort of figured that the meter might be off.
The issue I'm facing now is that the TDS meter is back to flashing 99 at all times and the phosphates directly from the clean water output are reading 2ppm, on an API freshwater tester, which is the same ppm reported in my local tap water. My DI resin is still mostly brown, its the color shifting variety, and I didn't think a membrane would need to be replaced so soon. Does any one have any advice, or have dealt with something similar?

I've heard this before with these units. Try a search for aquamedic here and see if those threads pull up. I could be wrong.

 

[Randy Holmes-Farley Verified Community Expert]

Phosphate gets through relatively easily compared to other ions, but those levels are zero concern being far, far less than you add each day in foods.

I discuss that here:

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 watercould 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.

 

[Tangdora]

Are you on well water. If your resin is brown it may be spent already. Brown/gold typically mean means exhausted. Blue/green would indicated in good health.