Is there any test we can do chemically or any way to prove that the water is safe for reef aquarium and has passed thru reverse osmosis de-ionized (RODI) filters before adding salt mix .Some people suggested me to get water from walmart,whole food stores coz they sell rodi water cheaper than fish store ($1/Gal),has anyone ever brought that water and then added salt mix and used for a reef tank ??? Thank you.

Everything you test if for should be around zero, except for pH which should be approx 7.

As long as hardness, nitrate, phosphate, ammonia (and anything else you have a test for) is approx zero, then it's practically pure water. Add your salt mix and be happy. If you get significant readings for any of those, then it's not pure RO water, and you are being ripped off. pets question and answers,www.5d2d.com

Some peoples tap water is clean enough to mix for reef tanks, but most tap water has all sorts of random nitrate, carbonate, magnesium, calcium phosphate etc that messes with the exact chemical makeup of your salt water. If you start with practically pure RO water, then your end mix should be more exactly like sea water.

Ian

Edit for Ryan.
You are correct the pH will be 8.4, AFTER adding the marine mix to the PURE water. The pure RO water that he is starting with should have a pH around 7. If you take pH 8 water, it already contains dissolved minerals, add the marine salt mix to whatever random chemicals are in that water, and you get more Total Dissolved Solids than should be in sea-water, and they may be in the wrong proportions. The Question is about testing RO water, to see if it's pure, not testing sea-water.

NO the answer above is wrong... If you have a reef tank, your pH should be between 8 an 8.4 ideally closer to 8.4 and if you test for calcium after adding salt mix it should be around 450. But you want your ammonia, nitrites, and nitrates to be at zero.

pets question and answers,www.5d2d.com

But to answer your question most LFS will do a free testing on the water and also they sell little boxes of test strips so you can test everything individually.