Distilled water PH SHOULD be 7, but thats not always the case, thats why you always gotta be checking you PH, always, from container to container. Anyways, get some water, take its PH down to 6.5 and saturate your growing medium with it, then let it sit for 15 minutes. After that, take the PHed water and pour it through your pot untill the runoff water is down around 6.5. With 2.5 gallon pots, you will need to flush each pot with at least 2.5 gallons of water. Really though you'll probably end up using about 5 gallons per pot. To make sure the ph is on, after I have given a thorough flushing I like to let my plants sit for about 20 minutes, then I give em just enough water to give me a little more runoff water which I then PH. This is kinda like your final PH, if its good, then you're set, if its not, then repeat the process untill your ph is right around 6.5.