New RODI / possibly first corals?

Just received my new spectrapure yesterday, and I was wondering how long I need to wait for old trace nutrients and minerals to be removed enough to add coral . I have a 55 gallon w/ 10g sump and 10g fuge (width under my stand won't allow any wider). I usually buy rodi from my lfs (6TDS) when I can, or distilled from a Culligan unit near my house ( 3 to 9 TDS) tested myself with HM TDS meter but occasionally because of funds have had to use tap water(200 to 350 TDS) about once every other month for topoff. Tank has been setup since September 2010 with 2 clowns and 1 bicolor blenny. With the tap water added I hold steady at 0 ammonia, 0 nitrites and 15 to 20 nitrates. I run tests weekly on an extremely old API kit, but I haven't purchased a new salifert kit because I haven't had the need. I've been planning forever to get corals starting with softies (zoas, shrooms, and leathers.) I've done 1 10 % water change with rodi so far and plan on doing 25% over the weekend. I'm hoping by the end of April with 10 to 25% water changes alternating weekly to purchase my first zoas, what do you think?

Oh yeah, I dose 10 ml vinegar in a DIY kalk dripper(2 tsp to 1 gallon daily.)