Need help diagnosing nutrient deficiency/lockout

This plant is 20 days old and started slowly losing its bottom set of leaves. They started curling and turning a bit yellow but all the rest of the plant seemed healthy. I thought it may have been a side effect of the fungus gnats I battled or even the superthrive over encouraging root growth. Over the last two days I tried giving it some more GH Micro for nitrogen as well as some H202 to help with the drooping and this is how the plant looks today. There have been no PH fluctuations either. After scanning through a bunch of different articles about symptoms of deficiencies I am leaning towards calling this a magnesium lockout. My water is around 300 PPM after using my carbon filter and I have read that excess calcium may cause magnesium to be locked out. Even though the plant does not seem to be experiencing nutrient burn today I plan to change the reservoir to plain Phed water until I can solve the problem (I’d rather lose some nutrients down the drain than keep killing my plant.)

Is anybody else able to give their input on what they feel the problem might be? Complete details of my grow are available in my grow journal which should answer most questions you have (or feel free to post them.) They are in a solution of about 300-350 PPMs of nutrients, a 1 to 1 ratio of GH micro and tigerbloom with a couple drops of superthrive.

My poor plant is screaming for me to kill it lol. In all seriousness though it doesn't seem to be life threatening just yet. I feel bad that this happened so close to me topping it though, that's a lot of stress.

Actually im a tard, i thought i posted this in the marijuana plant problems section >.< If any mod sees this feel free to kick it over there. Danke!

bumping for the night crew

So my local hydro store guy said I can use cal/mag, but I was under the impression that unless you are using RO water that most tap water has more than enough calcium. I'm going to be trying it out and will update this thread when I notice any results. I also picked up some epsom salt just in case.