Now you got them in a good place.
I so often hear people give advice that you shouldn't feed baby plants for two weeks after they sprout but that's if they're growing in soil and have some nutes available. Then it's decent advice. For DWC, and any hydro grow, some nutrients must be in the water or they are starving within a few days after they use up the tiny amount of food they are born with.
I'm very surprised to hear that your tap water comes out at such a low pH. Is this from a municipal/city source or your own well or something? Towns that have lower than 7pH water add something like calcium carbonate to the water to get it above 7 so it won't corrode pipes, pumps and other metal components of their system. You are calibrating your pH pen properly I assume.
Try testing the same glass of water 3 times. Make sure it is at room temp and turn the pen off after each test and wait a few min or longer before the next test. If any of them are more than 0.2 points off then you might have low batteries or the probe may fading or the reference electrode may be dirty or contaminated. They have 2-part cleaning solutions or you can soak for a while in low pH, say 4, then higher pH, say 10, solutions to help correct any unbalance. Soak overnight in each with a good rinse with tap water between. Between uses I store mine in proper storage sol'n and it greatly prolongs the life of the probe and helps keep it accurate.
I've been using the pH Perfect nutes for a few years now and honestly haven't bothered checking the pH since. The base nutes don't cost any more than others and I'm not sucker enough to pay through the nose for all the supplements. Base plus Rhino Skin and Big Bud is all of AN stuff I've used for almost 17 years in soil/soilless and DWC. Never even used any CalMag until few years ago and never saw Ca deficiency tho did see slight Mg deficiency that I added epsom salts to correct. Got me some Bud Factor X to try out. Had to order it in here in Kelowna while visiting and they gave it to me as a free sample. Stuff is around $80/L so my 500ml bottle would have been $40 or more. The new organic hydro nutes from AN aren't available yet in Canada but hoping they get approval here so I can try that in DWC.
Good luck!