My ICP tap water test just came in

[Ferry]

I am wondering if anyone uses tap water and remove the unwanted "water elements" and use it for reef. I noticed all useful elements such as calcium, magnesium, potassium, strontium, etc. are stripped of with RO DI. Thoughts anyone?

[DaJMasta]

While possible, I think the fundamental problem in trying to is the difficulty in targeting specific elements for removal.  One part of ICP is that it's analyzing the ionized components of the molecules present, so you don't necessarily know the molecular form of what these quantities came from (though you can infer it from knowing the chemical processes that carry it in the water in the first place), and then you need to very specifically target those problematic molecules for removal.  Since sorting nanomachines aren't a thing, that could be very selective fine sieving (like an RO membrane with a specific pore size), chemical additives, flocculants, or other methods, and probably several of them to get everything needed out and then remove the residual.  What's more, there's a chance that those problematic molecules actually contained the majority of the element you wanted to keep, or that the ones you want to keep are locked up in a molecule that isn't usable by reef creatures.  Now the last one is probably least likely given that it's in water, but still, your resulting solution of desirable elements/molecules would then likely not reflect the replenishing needs of your tank, so you'd need still more processing of it until it was in a form you could actually dose.

Chemistry is tricky.  The right stuff can be right there, but unless you can isolate it and add it in a controlled manner, you're just dumping stuff into your tank you can't really account for and hoping for the best.

[Origami]

Also, keep in mind that many of those "desirables" are measured (above) in parts-per-billion and are a fraction of what you would like to see in your reef. For example, 705 ppb Magnesium is the same as 0.7 ppm out of the target zone around 1300 ppm in a reef aquarium. 

 

These additional components are largely not controlled by your local water authority (unless some start to exceed some water quality standard), so their concentration may vary over time.

 

The purpose of using RO/DI in mixing your water is to start with a somewhat known baseline to which you can add your mix in order to maintain some consistency in the water that you use in your tank.