Still having some problems with these

My plants are now in 50L Pots in coco. They are receiving 1feed a day for 1min. The nutrients are Canna made up to 700ppm. Room temp is between 16 and 27 degrees C. Humidity is fine. The soil is moist with a dry layer on top. I'm seeing some white salt build up on the soil for some reason. Lights are 4x600w. I cannot work out why I'm having this problem although it does appear to be lessening over time. I treated them with Fungorid just in case about 2 weeks ago. The leaves pictured are the worst I could find and range from the very bottom of the plant to the middle. Top leaves show only minor yellow on the tips.

1 feed a day for 1 minute is not nearly enough I don't think. 27C is too warm. that's like 81F, which is a little too warm, but not terrible for indoors. you got a fan running?
is this flood and drain? I don't do that but shouldn't it be more like 1 minute every 5 or 10 minutes? or 15 minutes flood time every hour? your feed rate doesn't sound like it's enough.
your feeding like in soil? never heard anyone do this before. usually in soil it's feed a water fert mix and let the soil dry out, then either water only or water fert mix if necessary. you use both coco and soil, which one? are you using a mix of both? seems like coco by itself tends to create problems.

I'm feeding in Cocos only because it retains so much moisture I don't have to feed as regularly as with soil coco mix or perlite. Temp is nominal for the room area and I have a large extraction fan that changes the air out regularly enough. Have included a pic but couldn't do it under normal light. Gives an idea of space. 3x3m total room.

N Deficiency?