Should I check only EC or PH is also important?

Yes you always need to test the PH. Also depending on how much you payed for your ph pen. You will need to check its calibration. If you don't have the calibration kit then your pen should calibrate to at least 6.86. Which your cheapest bottle water has a palette approved ph of at 7.0 or just below so this will at least ensure your in the correct range, but still calibrate that thing and test it every time you add nutes. Especially if your using ones with high nitrogen from ammonia sources such as a fish fertilizer or fresh guano. Most nutes are very acidic so you will need to raise the PH. If you don't have ph up or down you can use baking soda as your base or alkaline to raise the ph add only a half a teaspoon at a time. Shaking the container and letting it sit for 2 minutes before testing. As for ph down you can use white vinegar but only a drop at a time just one drop will lower the ph by at least .5 or more so be careful. Also if your using an additive like recharge or great white then is the only time you do not want to lower the PH with vinegar will kill the microbial life in the jug before you add it. Recharge and great white both will raise the ph only slightly but it's fine and infact 7 or 8 is safe and habitable for microbes to live between 5.8 and 8. Lastly and again and again tell yourself this PH PH PH the plant itself can only up take nutrients in the very same safe range for microorganisms to live optimally 6.5 for soil. 6.2 for soilless and 5.8 for hydroponics. Oh and before I go be sure to look up and get yourself a chart of the big 4 mobile nutrients and their deficiencies because the bottom of your plants leaves are yellowing and that's a definite sign it's in need of more nitrogen. Happy growing my friend.