Does the water filter lower the PH, or do you adjust it? I never used one before, so I don't know. Anyway, the 'golden number' for soil growers is 6.8. That's the happy medium in which all nutrients are absorbed most efficiently. You can go lower than that,(6.5-6.8 is best) or even a little higher, but if you keep it close to 6.8 you'll never have a problem with PH. Soil tends to get acidic over time, so feeding/watering with 6.8 will help keep it more neutral too, longer.
Anyway...maybe it is your soil that's causing the problem? Have you checked the PH on that? If you don't have a soil tester, and only have a tester for liquids, you can throw a handful into a cup of water and mix it around, then check the water.(mix in alot of dirt, like 50/50. At least 25%) Also, as long as the PH is fine, that Shultz stuff should be fine.
Now that I'm awake, I'm changing my original answer.(sorry about that.LOL) After reading about your soil already having nutrients, then adding the MG twice in 1 week(too much for such a small plant), I'm starting to think it is fert burn. When you mixed the MG fert, you said you used 1/4 strength. Was that 1/4 strength for inside plants? That's only 1/2 of one of those tiny scoops on the small end of that green scooper that comes with the fert. That's all that you need for a plant that size,(that stuff is strong) and once a week should be plenty until the plant gets bigger.
Do you have a PPM(TDS) tester? If not, pick one up off Ebay. They're only $20, and work pretty good, much better than the cheap PH testers. If you want to play it safe, mix the MG to around 50 PPM for the next feeding(when the plants is visible recovered, and growing well). Then by the next week, increase it to 75 PPM, and then finally to 100 PPM.(a little higher is okay, as long as the plant is big enough to handle it) And make sure it's PH balanced first. I don't want to sound like I'm preaching, but most people underestimate how critical PH is, until they fuck it up and experience it for theirselves. I had to learn that way myself.LOL
Also, if you just flushed it, I would wait at least 5 days before giving it any more fert. You should see healthy new growth coming soon, so wait until after that, because that will let you know you have the problem under control. Also, I don't know how big those pots are, so maybe I'm underestimating the plants size and age? Either way, what just told you will work fine. It would also be a really good idea to pick up some 'cannabis specific' fert with trace nutrients, and use as directed. It's engineered differently for a reason....cannabis has different nutrient requirements than the generic ferts provide. You can get a small bottle off Ebay for less than $15, and will be happy you did. Best of luck.