Testing RO/DI water

[jporter]

I have my own BRS RO/DI system and it's been great for top off, and saltwater. My question is how to test it for phosphates, and any other pertinent parameters. Will my traditional saltwater test work for water right out of the RO/DI output?
I've recently started fighting a small cyno/algae outbreak. It has improved since I replaced my bulbs, and I've made some improvements with water flow. Not sure if the addition of a couple jawfish could have an impact due to substrate digging. What is the determining factor on when to replace the RO/DI filters? I'm sure this was all spelled out in my filter instructions but I have no idea where they are. Thanks in advance. Jeff

[pop0tart]

I believe you want to get a TDS meter and use that as a guide for when to replace filter membranes. I believe phosphate is an anion, and should therefore be removed by a properly functioning DI resin.

[fishchef]

Just test your saltwater mix before adding during partials for PO4. Should be zero if all is working properly. Reserve saltwater can be tested as you would for your tank.

[Randy Holmes-Farley]

IMO, there is never any need to test RO/DI for phosphate if the TDS reads 0 ppm. Even if you detected a little, and even if it were real, that small amount (say, 0.05 ppm) is insignificant in relation to the aquarium and the phosphate inputs from foods.

I discuss that here:

Phosphate and Food
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.