Mate, the CaCO3 in water is anything but "available"... it's weakly soluble at best, most is locked into molecular form, still bound together...
... free ionic Ca++ is what the plant needs, and the bitch (besides coco's bitchiness with Ca) is that Ca++ is wildly reactive with that double positive charge! Unless there's something to chelate it, it's likely to go right back into carbonated form or worse...
Amino acids and humic-fulvic are you friends here, h-f especially... Excellent stuff regardless of your medium/method, BTW...
...nute makers will chelate their stuff with other synthetic types, but that only goes so far...
An in-pot pH reading will elucidate further... acidity helps with Ca availability, up to a point... Coco likes to be upper 5's-low 6's pH...past this it starts to get iffy in general...