I love Neptune's Harvest. I use kelp and crabshell in my mix.
Usually, I have a bucket with some kelp and alfalfa meal soaking for nutrient tea.
Even with the crab shell in the mix, I still get what look like Cal and Mag deficiencies near the end of flower.
I've had success adding kelp meal and crab shell to EWC, wetting it, then letting that mixture sit around. When the crab shell is all broken broken down I top dress with this mixture. Not exactly the same as your tea, I know.
Molasses has some calcium and magnesium, but all I can find is percentages of R.D.A. for people, not actual content.
Bloom bat guano has some calcium. And they're both water soluble, but I've burnt plants using them. So I started using the kelp/crab top-dress. This has burn some plants too. I've had way better results with the "loaded EWC" topdress.
If you use powdered Azomite, that stuff is sorta water soluable, like Nestle Qwik powder.
I know there are also Crab Meal (not crab shell?) and Fish Meal. Maybe these meals are more water soluable, but good-lord, fish meal tea sounds stinky. I don't have any experience with that.
I've added more fish-bone meal and gypsum to my latest batch of soil, maybe these will get me through without the topdress in the future.