CMOS
To be completely honest,there's not a phosphate kit on the market ( at least not a working man can afford ) that I WOULD recommend. The kits that's on the market now just wont read low levels of phosphate and it don't take much at all to really screw up a reef tank.And while I do like API for stuff like alkalinity and calcium and nitrates,I wouldn't even sale you their phosphate kit if you walked into the store I help at.Just not worth the cardboard it's packaged in.
At the LFS I help at,we've found that 90% of the time,phosphate kits will either read phosphates when there should be none,or vice versa.And we tested that out on RO/DI,distilled,tap,and carbon filtered water.
Another thing you've got to keep in mind too.The second you add anything alive to your tank,your introducing phosphates.All life produces organic phosphates which there is no test for anyway.
Something I would try.Take a sample of your new filtered water to your water company and see if they'll test it.Don't tell them it's RO water.See what results that shows before digging to far.If that shows phosphates,then you'll know it's your filter.If it's clean,then it'll have to be something either in your mixing barrel or from somewhere else.
Any chance of fertilizer dust getting into your water either from your clothes or from some farmer fertilizing his fields with dry bagged fertilizer? ( just grabbing at straws and throwing it out there.)