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I never gave it a second thought until someone mentioned not to use it for FW; the I went looking to validate the statement. So, no it's not a science experiment, I don't want to 'play w/ my numbers.
However if you consider your time to be worth something I would wonder what the true cost benefit number is for the ro/di filter once you factor in dosing, testing, mixing time and costs. For FW it sure isn't filter the water - use the water as compared to using bottled water as I do now which is open the 5g jug and siphon in.
...And for the record, it was Bob Fenner over at wetwebmedia whose posting first recommended mixing the two first in this thread. Credit where credit is due
I was thinking that after the initial test phase you would have a percentage that should "hold true" for most cases with enough *accuracy* that you would not have to worry about stressing the life in your tank from a big change in water mineral content and pH.
I simply agreed that it sounded like a good idea to mix the two [not claiming credit for anything] but even better if a reputed/respected member of the aquarium community espouses such ideas.