New RODI unit..should I expect 0 phosphates?

I just purchase a 90gpd rodi unity from spectrapure and have begun using it. I followed the instructions to get the ratio correct and ran 3 gallons through the unit (they recomend 2) before using the water, and I tested for phosphates and it registered at .025. First of all, is that concsidered high and should I expect it to be zero. I just switched from an RO unit because it was reading 2.0...definitely an improvement, I guess I just expected it to be 0

 

You cannot accurately measure the phosphates in ultrapure water with hobbyist grade test kits, this has been repeated over and over.

It takes lab grade instruments to measure individual ions or substances in RO/DI and distilled waters and this is why we as hobbyists use TDS meters which measure all dissolved solids together using electrical conductivity or resistivity.

If you do not have one, get a good handheld TDS meter and use it to measure the quality of your water, if it shows 0 TDS then you are good to go. Once the TDS starts consistently shwoing anything other than 0 TDS then it is time for a DI replacement.

 

Sounds good. Thanks for the response. I plan on getting a TDS meter, I just figured I would test phosphates since I have a test already. Being that it is brand new, I am assuming it is doing its job. Just my noobness showing through, I wasn't exactly sure what shows up on a TDS test :-[

 

so is there any point in measuring phosphates once in the aquarium or is that completely different?

 

A lot of foods can add phosphates back into the aquarium. so you would still want to test for it.

 

You can still add phosphates through other means to your DT besides water changes. One source is likely food.

 

Hobbyist reef type test kits are designed to test the saltwater in the tank.

TDS or total dissolved solids is a mathmatical measurement of everything electrically conductive present in the sample. It does not speciate or tell you any particular thing, only that something conductive is present and you need to do something about it if it is in your finished water.
One thing you want to wacth closely is the final RO/DI TDS since some contaminants like nitrates, phosphates and silicates are only weakly ionized or conductive. What that means to you is since they don't have much of a positive or negative charge they can be released by the DI resins once they start exhausting and end up in your treated water. Also since they are weakly ionized they do not show up well on TDS meters at very low levels and you could be releasing bad things by the time the TDS shows up much more than 1 or 2 and not even realize it. Change your DI when you first start seeing signs of TDS. Also keep up with your RO only TDS readings as this tells you the condition of your RO membrane, not just the tap and RO/DI but three TDS readings, tap, RO only and final RO/DI are necessary and why inline meters are not the answer since they are limited to two spots. With a handheld you can test anywhere including your ATO storage, bottled water, the LFS, vending machines, your buddies water etc.

 

Good to know thanks.

 

KK sorry I was a bit confused by the whole hobby test kits comment. You mean that fresh ro/di water really can't be tested but once in our tank and "dirty" our hobby test kits will work fine for checking for phosphates? (sorry new guy here ;D)

 

Yes.
Ultrapure water is very tricky to test as there can be so many interferences such as CO2 in the air, a speck of dust, dirty test tubes etc. You are testing something that is highly reactive, it is out of its normal parameters and trying its darndest to get back to its natural "dirty" state. It will attract anything it can to replace the ions that have been stripped out, even water spots on a test tube will alter readings. There just are not enough ions to measure so we use TDS which measures the ions that are present.