I'm not familiar with Canna's line of nutrients but took a quick look at their feed schedule. Their schedule is for RO water and they suggest ignoring your starting (tap) EC. Add .2 EC of your
CalMag and then 1 EC of your base nutrients (A/B) for a total of 1.4 EC (this is including your tap EC). Let this sit for a bit and see where your pH is. Adjust your pH to 5.8 and again, let this sit for a few hours and see if the pH drifts any. I'm curious if the pH of your feed is stable. Once you have a stable pH, water your plants until the runoff is the same EC as your input, in this case it should be 1.4 EC.
Feed like this for a few days and see if your plants react well. It's really a process of elimination, Just make sure everything else is in check (lighting, temps, rh, etc..)
Next time you might want to consider smaller pots but that's entirely your choice. 20 liters of coco seems like a bit much.
Another unknown is how well your roots are established, especially in a container that big. As I mentioned, I know every says you can't over water coco and that's true for the most part BUT only if your roots are well established and healthy. In containers your size, dont be nervous to let them dry out for an extra day or so, just make sure sure water them well to keep any salt buildup in check.