1 week? did you dose with ammonia? if not, then you were just running water through your system, not "cycling" - the process takes anywhere from 6-12 weeks, and depends on a number of factors..
the nitrification process does indeed cause the ph to drop, i use "hydrated lime" to buffer, and have used shellgrit and eggshells in my growbeds to keep the ph at about 7.2 or so..
when your ph drops below 6 (the api test kit only goes down to 6) your bacteria colonies can "crash"....(not a good thing)
and then you basically have to begin cycling all over again..