you need a test kit to test for phosphate .
it would be safer just to use distilled water till you can get a RO
Like Matt said why risk adding fish to see something bad come about .
your spending all that money building up your tank why risk screwing it all up with tap water . I did that very same thing with my first tank and lost some costly fish , it's not worth the risk considering 79cents a gal
for distilled is a small cost to be sure the fish are safe , Most of the nicer salt water fish are in the $50 range