This app was mentioned in 3 comments, with an average of 1.67 upvotes
I would say magnesium 85% sure. Increase nutes and water with food with every watering, coco flushes easy and won't hold food/salts like soil will. I would suggest hitting it with a full dose especially if you're using clean foods like Canna. I made an app that helps people identify nutrient deficiencies it's called Dr. Jane and its free try it out and see if it come up with magnesium? Cheers and best of luck. https://play.google.com/store/apps/details?id=com.travisecarney.deficiencydiagnosisads
Looks potentially like a micronutrient deficiency, make sure you're adding enough food to your water and that you're pH adjusting it correctly usually right around 6 is fine. If you want a more in depth diagnosis you could check out Dr. Jane https://play.google.com/store/apps/details?id=com.travisecarney.deficiencydiagnosisads
Luckily most deficiencies can be treated in a similar manner. Make sure that you are pH'ing your water to about 6.0 (if using conventional fertilizers) and make sure that you are using a fertilizer that is complete with everything the plant needs. Something like Flora Nova from GH is a good simple solution. I would also recommend to use an enzyme or mycorrhizae product which can help with the availability of nutrients. Also do a double check on your hi and low temp and humidity. Here is a link to a free android app that I made that helps with the diagnosis and treatment of plant deficiencies it's called DR. Jane