Agreed on mag def: regardless, it is time for a transplant to a 2-3 gal pot with a good soil mix, and then feed with calmag and veg nutes on next watering. Pre-soak your new soil with a light nutes mix first for ~24hrs, it should be moist not soaking wet when transplanting. Wait for your plant to get thirsty and then transplant your ~dry rootball into the moist soil surrounding. Let it take to the new soil for a day or two before your next water/feed.
I use promix/sphagnum/coco/perlite/worm casting mix with Fox Farm nutes trio as my base. For ~3gal of water, I usually mix 2 cap-fulls of calmag, 1 cap of grow big, and 2 caps of the big bloom. That gets me pretty close to target EC and Ph is never an issue. I also add Orca beneficials the make the roots happy. You can use Mykes powder when you transplant to help with root health also.