Whatever you decide to do, if you're growing in living soil/organic, you need to be super careful about what you put in the soil. When growing organic, you're not really so much tending the plant so much as you're tending the soil and the microbiome that it supports. Many synthetic nutrients/supplements (including Cal/Mag) will kill all the wonderful bacteria and fungi that you've nurtured. That's bad because in a living soil setup, the fungi and bacteria are what get the nutrients to the plant. If you kill them off with synthetic additions, you'll likely be sad. When growing weed, nothing good ever happens quickly. That's even more true when growing organic.
Do a quick Google search for 'sources or organic calcium for plants' and you'll find lots of options from dolomite lime and other items mentioned in other replies to simple things you have in your kitchen like egg shells. Although this will take a little bit of time to break down and become available for the plant, you'll be preserving your soil biome. And from the looks of your plants, you still have several weeks before harvest.
For future living soil grows, it is important to know that the 'water only' technique is only partly true. You still need to add organic teas and do top dressing so that you're keeping enough nutrients in the soil. If you re-run genetics, you'll hopefully take notes so you'll know that this particular strain needed more calcium or nitrogen or whatnot so you can make sure that it's in the soil (organically) and ready to go for your plants. For example, my Blue Dream needs LOTS of nitrogen in veg stage. I didn't know that the first time, so I ran into a deficiency. But the next time I ran those genetics, I was able to be proactive and make sure that I supplemented with N before it was needed since, as I said things take time, especially in organic soil grows.
Be patient, fix what you can, and you'll be prepared for the next grow!
