pH:

PH is critical to the plants ability to uptake nutrients and additives. In soil PH should be in the range 6.0 to 7.0, in hydroponics PH should be in the range 5.8 to 6.2. Falling a little outside of this range won't cause a problem but if you go too far you risk locking your plants out so they cannot absorb nutrients properly.

In recirculating hydroponics PH rises as the nutrient goes around the system so some schools of thought say you should start the PH lower at around 5.5 to allow for this. In a perfect world you would adjust the PH every day or so to 5.8-6.2 but if this isn't possible starting at a lower PH around 5.5 in recirculating systems would be a good idea.

EC:

EC (electrical conductivity) is a measure of the strength of your nutrients in water. To measure EC you will need either an EC meter or BlueLab EC truncheon. Some growers rely on the dosing instructions on the back of the bottles but this leads to a lot of guesswork, accurately measuring and adjusting your EC levels is the key to successful growing so get an EC measuring device, they will pay for themselves, trust me!

As a good rule of thumb anything under EC-1.0 is too weak and anything over EC-2.0 is too strong. Good growers will start their rooted cuttings and small plants on EC-1.0 and then ease the strength up over time so they get to around EC-1.6 by the time they go into flower.

Different nutrient manufacturers will recomend different EC levels and it can be hard to know what to do. Keep an eye on the leaves of your plants, if just the tips show a bright yellow discoloration this is called "Tip Burn" and your feed is too high, if your leaves go completely yellow inexplicably your nutrients could be too low.