i add the zone 24 hours before a rez change
im sure theres a lil left in there after dumping the old stuff but unless your running a very very small amout of water, or not doing a good flush in between weeks, then it could be a problem
hydro shop had a high power scope out and told us to bring water samples in to check out if we wanted. theres something alive in there. a little drop of zone and nothing.
again im only running my zone on day 7 of 7, days 1-6 no zone.
i really should be telling people a good flush is very important with this method, but i kinda assumed that was a gimme
smoke stack you are saying that you add zone on day 1 of 7 right? this is what most people do