Toxicity, overwatering or deficiency? What have I done wrong?

OVERWATERING!!!!!!!

The drooping leaves make me think your watering is a bit off - I certainly don't see any sign of a deficiency and so am a little perplexed as to why you're giving her calmag... if you maintain the pH level you've got, the regular nutes will have enough calcium and magnesium in them and unless you've given them a bit too much nitrogen and caused a lockout, they shouldn't have any problems. If you have overloaded her with nitrogen (I don't see it), that will cause a lock out but the solution to that is not to throw calmag at her, you'd have to flush, then cut back on the nitrogen-rich nutes going forward... But droopy leaves like that indicate either over-watering or underwatering and if you're only giving them 1/3 gallon of water every day, this tends to make me think they're being underwatered. You need to lift the pot every day to see if it's lightweight and if it is, give her a drink (or nutes depending on where she is in your water/feed/water/feed schedule). Don't say "Oh, this is Saturday, I need to give her water." Let the plant tell you when it needs something. And when you water, make sure you're getting at least 10-20% runoff and check the pH of that runoff, too... If you don't get the runoff, you're enabling nutes and salts to build up in the soil which will eventually require you to flush them. Good luck!

you had just deeper problem, go same way to sort .... I honestly use calmag 3-5 times in all grow time. In soil, you should not add any mag , especially in big or constant quantities , there is overdose too ... not only lack or not possible to absorb aka bad conditions - either watering regime or ph ...

Magnesium deficiency, but before just dumping in calmag make sure your pH and watering is on par, because this can also cause issues with the plant uptaking it.

Ho! It's a magnesium deficiency, use cal-mag (from biobizz too for example) and she will be fine!