To give fertilizers or not?

Short answer is yes. When growing indoors your plant has needs that still need to be met and the only way for that to happen is for you to meet them. Personally in veg I give bate minimum if any nutrients as my plants grow because they are being transplanted every couple weeks and are being replenished with nutrients. By the time the plants are in flower they are in they're growing medium for quite some time now and have depleted most if not all of the available nutrients and this is the time they need them most. I highly recommend starting a low dose bloom feed and I say low because youre growing autos and they typically only require about half the amount of nutrients as photoperiod plants. I hope this makes sense and helps. Also as far as macro nutrients like calcium and magnesium, boron, iron etc. If you give your plants tap water then they will get plenty of these nutrients.