But what if the waterchanges are done with distilated water too?
They may say its "in the figure"of 5ppb but that means it could be 7 or 8 too.
Last but not least, i know alot of systems which are older than 5 years.
All together you will get a very different level after recalculating.
Test your water for po4, in a fish-only tank its very possible the level of it is too high for a good coraline growth.
Po4 slows down the growth of it.
The risk of using distillated water is that the installation used to produce it has alot of copper parts which could leave traces of copper in te water.
Personally i wouldnt take the risk.