no, I agree with the original suspicion
the charcoal after the DI
is the source of your phosphates
the aquasafe unit I ordered comes with a carbon post-filter, which is designed to "take the edge off" the DI
water, so it doesn't taste so terrible... but fish won't be concerned about taste...
I plan on just hooking the carbon filter up to the supply going to the drinking water pressure tank, which I'm going to re-route off the RO
output, and save the DI
output just for aquaria use
try unhooking your carbon post-filter, and retesting the ro
water for phosphates - that should either confirm or exonerate the carbon as the source of phosphate!