I mean... you CAN use tap water... but you should have it tested first for chlorine. You really don't want any chlorine in there, plants hate it. If you live in the city, there will almost undoubtedly be too much chlorine in it, or worse chloromide. At least with chlorine, you can let the water sit out for 24-48 hours and it will evaporate... chloromide however will not. You can take a sample of your tap water to your local aquarium and have them test it for you. I would also test for PPM. You don't want your water too hard. Honestly, I would just buy a couple 5-gallon jugs and fill them with RO water and get some cal/mag from your local hydroponic shop and put about 200 ppm's of cal/mag every 2nd or 3rd watering... you don't really need much. I don't even think your soil has a deficiency right now... probably just pH fluctuating too much... or lockout from too much salt build-up caused by your tap water. But your main issue right now is overwatering honestly. Seriously, just let that soil really dry out (to where the pot is super light when you lift it) before you water again. She'll love you for it.
Btw, you're not going to get either a calcium or magnesium deficiency from watering with bottled water one time. I use strictly RO water for the first 3-4 weeks of vegetative growth before I even feed them any nutes and never see any signs of cal, mag or any other deficiencies. And that's with Fox Farm Happy Frog soil which is not even close to being as packed with nutes as your time-released MG.