Schefflera arb., chlorosis, brown spots on new growth, 6-fingered instead of 7

It feels like I'm fighting an invisible enemy. My schefflera arboricola (named "the schefner") plant was the first to exhibit the symptoms, weaker new growth, rust spots, some yellowing - then it was my two MJ ladies, Samus and Faye. Next? Just yesterday, I noticed that the new leaves coming in on my Wandering Jew were brown/necrotic in the middle, wavy, weak, and it seemed like it had progressed a bit, to the second youngest set of leaves.

I'm fairly certain that my ladies and schef have/had fungus gnats, schef first - apparently the soil I used (which I really shouldn't have), MG organic, has a bad rep for coming with fungus gnats eggs in it. Could damage like that be attributed to those bothersome pests? I've had that schefflera for two years, root-bound in a tiny pot, but it never had a single yellow leaf or deformity that entire time... up until I transplanted 'im around a month ago (more recently that this started happening, though).

I don't know if this helps, but my ganja girls were also exhibiting signs of a cal/mag def, but I'm not sure how this would manifest in other kinds of plants - just MJ, haha.

Thanks for your time, when my plants hurt I kinda feel it in a way.

Forgot to say, applied a soil drench of AzaMax recently, seems to have helped a bit.

Sheff is a really hardy tropical. I only saw a small amount of damage. I wouldn't jump to any conclusions yet. Fungus Gnats? Do they carry disease? Idk. I didn't think they did. If it doesn't increase I think your prob is solved. Epsom salts for cal/mag def
One tsp/gal

I know, I just get paranoid about my plants - scheff survived in a tiny little pot in which the soil had been compacted and super-glued together (when I got 'im), and it still did fine, though it didn't start growing nearly as much until I transplanted it (prior to the random damage) into its new abode. Not disease, but the larvae eat the root hairs, drain energy from the plant and can cause visible symptoms stemming from nutrient deficiencies.

Then, naturally, the grower assumes the plant's deficient because the medium is lacking... so they add more fertilizer and then cause nute burn on top of everything else, because really it was just the larvae diverting nutes/energy. It's a conundrum, but I'm not worried about it anymore - you could drive yourself insane ruminating over such things.