I have this meter: http://cgi.ebay.com/Milwaukee-pH51-...yZ117435QQssPageNameZWDVWQQrdZ1QQcmdZViewItem I'm not sure if all pH meters are like this, but it takes at least a few seconds for the reading to stabilize. I've also had issues where I measure the pH and it's 6.8, but then the next day the same container is 6.1 or 7.5. I'm battling with a zinc deficiency right now (I suspect it's either due to pH fluctuations or possibly hard water - calcium?) I wasn't too concerned until I noticed that my G13 feminized seedling is starting to exhibit the same yellowing symptoms. I use tap water. I let it sit in a 1 gallon jugs for about two days (I shake it up once a day or so) to let the chlorine evaporate. Typically, the pH is about 6.1. I generally use two drops of pH UP stirred in really well to end up with a pH of about 6.7 or 6.8. I then immediately water with it. Am I supposed to let the container sit for awhile after adding pH up or down? General opinions?